Hy's Law: Identifying Potential Serious Drug-Induced Liver Injury

Hy's Law: Identifying Potential Serious Drug-Induced Liver Injury

Introduction

Drug-induced liver injury (DILI) is one of the most important forms of organ toxicity considered during drug development and pharmacovigilance. Although most elevations of liver enzymes do not progress to severe clinical liver injury, a small number of drug-related hepatic events can result in acute liver failure, liver transplantation or death.

The challenge for pharmacovigilance and drug-development programmes is therefore not simply to identify elevated liver enzymes. It is to determine whether a pattern of laboratory and clinical findings indicates a potential risk of serious drug-induced liver injury.

Hy's Law is one of the most important concepts used for this purpose.

Hy's Law describes the observation that a drug capable of causing hepatocellular liver injury, when accompanied by an increase in bilirubin without a better alternative explanation, may be associated with a risk of serious liver injury. In drug-development settings, the occurrence of convincing Hy's Law cases is considered an important signal of a potential drug-related risk of severe hepatotoxicity.

The concept is particularly important because severe idiosyncratic DILI is uncommon. A typical clinical development programme may contain too few exposed subjects to observe actual cases of acute liver failure. Investigators therefore need to evaluate less severe biochemical signals that may indicate the potential for more serious injury. FDA's guidance on premarketing evaluation of DILI describes this principle and provides a framework for evaluating liver-test abnormalities during drug development. [1]

Hy's Law should not, however, be treated as a diagnostic test that predicts liver failure in an individual patient. It is better understood as a drug-development and safety signal concept that identifies a pattern requiring careful medical evaluation and consideration of alternative explanations.

The distinction matters.

A patient who meets biochemical thresholds associated with Hy's Law is not automatically experiencing liver failure, and a patient who does not meet those thresholds is not necessarily free from clinically important DILI.

Hy's Law is therefore one component of a broader medical assessment that includes the pattern and magnitude of liver-test abnormalities, clinical findings, time course, concomitant medications, underlying disease, competing causes and follow-up.

For pharmacovigilance professionals, understanding Hy's Law requires both knowledge of the numerical thresholds and the clinical reasoning behind them.

Learning Objectives

After reading this article, the reader should be able to:

explain the origin and purpose of Hy's Law;
distinguish Hy's Law from a diagnosis of DILI;
describe the laboratory pattern associated with a potential Hy's Law case;
explain why ALT, AST, alkaline phosphatase and bilirubin are considered;
distinguish hepatocellular, cholestatic and mixed patterns of liver injury;
explain the importance of the R-value;
identify the role of competing causes;
understand why Hy's Law is particularly important during drug development;
recognise the limitations of Hy's Law;
understand how Hy's Law cases are evaluated in clinical trials and pharmacovigilance;
explain how eDISH can support evaluation of liver-safety data;
understand what a safety physician should consider when reviewing a potential Hy's Law case.

Understanding Hy's Law

What Is Hy's Law?

Hy's Law is a clinical drug-safety concept used to identify a pattern of liver-test abnormalities that may indicate a risk of serious drug-induced liver injury.

The traditional biochemical pattern involves:

elevation of ALT or AST indicating hepatocellular injury;
elevation of total bilirubin;
little or no evidence that the bilirubin elevation is simply explained by cholestasis;
no more likely alternative explanation for the combination of findings.

The commonly used biochemical thresholds are:

ALT or AST at least 3 times the upper limit of normal (ULN);
total bilirubin greater than 2 times the ULN;
absence of a substantial initial alkaline phosphatase elevation suggesting cholestasis;
absence of another more likely explanation for the combination of findings.

The thresholds should not be interpreted in isolation.

The central concept is the combination of hepatocellular injury and jaundice or bilirubin elevation without a more likely alternative explanation.

This is why a patient with an ALT of 3× ULN and bilirubin of 2× ULN does not automatically constitute a Hy's Law case.

The medical reviewer must establish whether the overall clinical and biochemical picture is compatible with the concept.

Hy's Law Is Not a Literal Law

The term "Hy's Law" can be misleading.

It is not a biological law in the same sense as a physical law.

It is a safety observation associated with Hyman Zimmerman and subsequently developed into an important regulatory concept for evaluating potential serious hepatotoxicity during drug development.

The observation is based on the increased concern associated with hepatocellular injury accompanied by jaundice in the absence of a more likely alternative cause.

The concept became particularly influential because severe idiosyncratic DILI is sufficiently uncommon that clinical trials may not observe enough cases of acute liver failure to directly estimate the risk.

Instead, investigators use characteristic biochemical patterns as early indicators of potential serious risk.

Robert Temple's discussion of Hy's Law in the FDA context describes its use in predicting serious hepatotoxicity and illustrates its importance in drug-development safety evaluation. [2]

Why Hy's Law Matters

The importance of Hy's Law arises from the relationship between liver injury, bilirubin elevation and clinical outcome.

A mild isolated increase in ALT is relatively common in clinical development.

Such an increase may result from:

transient drug effects;
underlying disease;
concomitant medications;
exercise;
alcohol;
intercurrent illness;
laboratory variability;
other non-drug-related causes.

Most such abnormalities do not progress to liver failure.

The combination of significant hepatocellular injury and bilirubin elevation is more concerning.

Bilirubin elevation indicates impaired hepatic handling of bilirubin or another process causing hyperbilirubinaemia. When it occurs in the setting of hepatocellular injury and is not adequately explained by obstruction, haemolysis, pre-existing disease or another cause, the possibility of clinically important DILI becomes substantially more important.

The clinical concern is therefore not simply enzyme elevation.

It is the pattern.

The Scientific Basis of the Concept

The liver has substantial functional reserve.

Many forms of hepatocellular injury can therefore produce considerable biochemical abnormalities without producing immediate hepatic failure.

ALT and AST are markers of cellular injury rather than direct measures of liver function.

Bilirubin provides different information.

A significant bilirubin elevation occurring together with hepatocellular injury can indicate a more substantial disturbance of hepatic function or bile handling.

This distinction explains why ALT and bilirubin are often considered together when evaluating potential severe DILI.

The concept is also influenced by the observation that serious idiosyncratic DILI is rare. A clinical-development programme may detect numerous subjects with elevated transaminases but very few patients with actual liver failure.

Consequently, identifying a pattern associated with a substantially higher risk is useful for regulatory decision-making even when the absolute number of severe outcomes is too small to estimate directly.

Hy's Law Is Primarily a Signal Concept

Hy's Law is most useful when considered as a signal of potential serious drug-related liver injury.

It is not intended to replace clinical diagnosis.

A potential case should trigger a structured evaluation of:

laboratory results;
clinical symptoms;
temporal relationship to drug exposure;
concomitant medicines;
underlying disease;
viral hepatitis;
biliary disease;
alcohol exposure;
ischemic injury;
autoimmune disease;
other competing explanations.

The final medical assessment depends on the totality of evidence.

Hy's Law and Drug-Induced Liver Injury

Hy's Law Versus DILI

Hy's Law is related to DILI but is not synonymous with DILI.

DILI is the broader clinical condition.

Hy's Law identifies a particular pattern of DILI that raises concern for serious outcome.

A useful conceptual relationship is:

DILI

↓

Determine pattern of injury

↓

Hepatocellular / cholestatic / mixed

↓

Assess severity and clinical findings

↓

Assess bilirubin and other markers of hepatic function

↓

Evaluate competing causes

↓

Determine whether a potential Hy's Law pattern exists

The distinction is important because DILI can occur without satisfying Hy's Law.

Conversely, a patient may meet biochemical thresholds that trigger a Hy's Law evaluation without ultimately being determined to have drug-induced liver injury after alternative causes are investigated.

Hy's Law Versus Causality Assessment

These concepts should remain separate.

A patient may meet biochemical criteria that trigger investigation of a potential Hy's Law case.

The subsequent medical evaluation may conclude:

drug-induced liver injury is likely;
drug-induced liver injury is possible;
another drug is more likely;
an alternative disease explains the findings;
the available evidence is insufficient.

Therefore:

Hy's Law pattern ≠ automatic drug causality

and:

Drug-induced liver injury ≠ automatic Hy's Law case

The medical reviewer should preserve this distinction in case narratives, clinical-trial listings, aggregate analyses and regulatory communications.

Hy's Law Versus Acute Liver Failure

Hy's Law indicates concern for potentially serious hepatotoxicity.

It does not itself constitute a complete severity grading system.

Severity assessment should consider factors such as:

hospitalization;
hepatic failure;
coagulopathy;
encephalopathy;
ascites;
transplantation;
death.

A patient can have a biochemical pattern compatible with Hy's Law without developing liver failure.

Conversely, serious liver injury can occur through pathways that do not fit the classical Hy's Law pattern.

This is one reason Hy's Law should be used as part of a broader liver-safety assessment.

Individual Case Versus Drug-Level Signal

An important distinction is often lost when Hy's Law is discussed.

There are two related but different levels of assessment.

Individual-subject level

The safety physician evaluates whether an individual subject has a pattern compatible with potential Hy's Law and whether another cause is more likely.

Drug-level population assessment

The sponsor and regulator evaluate whether the overall clinical-development programme contains convincing cases suggesting that the drug itself has a risk of serious hepatotoxicity.

A single subject with ALT and bilirubin elevations does not necessarily demonstrate that a drug has a Hy's Law signal.

Conversely, several well-characterised cases can become highly informative even when each individual case contains some uncertainty.

The drug-level assessment therefore considers the complete clinical-development dataset.

Formal Hy's Law Criteria

The FDA Framework

The FDA's 2009 guidance provides a specific framework for identifying potential Hy's Law cases during drug development.

The traditional framework contains three principal elements.

First, there should be evidence of hepatocellular injury in the treated population, generally reflected by more frequent ALT or AST elevations greater than 3 times the ULN compared with the control group.

Second, among subjects with aminotransferase elevations, some should also develop total bilirubin greater than 2 times the ULN without an initial substantial increase in alkaline phosphatase, generally expressed as ALP greater than 2 times the ULN.

Third, there should be no other explanation for the combination of aminotransferase and bilirubin elevations, such as viral hepatitis, pre-existing or acute liver disease, or another drug capable of producing the observed injury.

This framework is important because Hy's Law is fundamentally a drug-level safety concept, not merely a laboratory threshold applied to isolated patients.

The FDA guidance describes the purpose of this approach as identifying signals of potential severe DILI during drug development, recognising that severe DILI is uncommon and may not be directly observed in a clinical-development programme. 3

The Three Elements of a Convincing Hy's Law Signal

The concept can be understood as three linked questions.

The Role of the Control Group

Hy's Law is particularly powerful when laboratory abnormalities occur more frequently in subjects receiving the investigational drug than in an appropriate control group.

Background elevations in ALT and bilirubin occur in clinical-trial populations for reasons unrelated to the investigational treatment.

A control group therefore provides important context.

The analysis should consider:

treatment group;
placebo or active comparator;
baseline abnormalities;
exposure duration;
laboratory monitoring;
concomitant medications;
underlying diseases;
observed liver events.

A pattern occurring equally in treated and control groups may have a very different interpretation from a pattern concentrated among subjects receiving the investigational drug.

Absence of Significant Cholestasis

The traditional Hy's Law framework includes the absence of a substantial initial ALP elevation.

The purpose is to distinguish hepatocellular injury from predominantly cholestatic injury.

This distinction matters because bilirubin elevation can occur in cholestatic disease through impaired bile flow.

A patient with:

marked ALP elevation;
bilirubin elevation;
relatively modest ALT elevation;

may have a cholestatic process rather than the classical hepatocellular pattern underlying Hy's Law.

The reviewer must therefore examine the complete biochemical pattern.

Emerging R-Value-Based Definitions

Recent research has examined whether an R-value-based definition may identify hepatocellular DILI more effectively than the traditional requirement for ALP below 2× ULN.

A 2025 study compared a normalised R-value of at least 5 with the traditional ALP <2× ULN criterion among clinical-trial DILI cases with ALT or AST at least 3× ULN and total bilirubin above 2× ULN. 5

This work is important because it illustrates that the scientific definition of a hepatocellular Hy's Law pattern continues to be evaluated.

For an educational article, however, the established FDA framework should remain the primary reference point, while newer R-value research should be presented as an evolution of the methodology rather than as a replacement for the regulatory definition.

Laboratory Evaluation

Core Liver-Test Variables

Four laboratory measures are particularly important when evaluating liver injury:

alanine aminotransferase (ALT);
aspartate aminotransferase (AST);
alkaline phosphatase (ALP);
total bilirubin.

They do not provide identical information.

ALT is a relatively liver-specific marker of hepatocellular injury.

AST can also increase with hepatocellular injury but is less specific to the liver because it is present in other tissues.

ALP is useful when evaluating cholestatic injury, although ALP can also originate from non-hepatic sources.

Total bilirubin provides information about bilirubin handling and becomes particularly important when evaluating potentially serious hepatocellular injury.

The interpretation therefore depends on the pattern and combination of laboratory abnormalities rather than one isolated value.

ALT and AST

ALT and AST are aminotransferases released into the circulation following cellular injury.

ALT is generally more liver-specific than AST.

An increase in ALT is therefore particularly useful when evaluating possible hepatocellular liver injury.

AST may also be elevated in DILI but can increase because of:

skeletal muscle injury;
cardiac injury;
strenuous exercise;
haemolysis;
other non-hepatic conditions.

For this reason, an isolated AST elevation should be interpreted cautiously.

In the context of Hy's Law, ALT is generally the preferred marker when available because of its greater hepatic specificity.

Alkaline Phosphatase

ALP is particularly important for distinguishing hepatocellular from cholestatic patterns of liver injury.

However, ALP is not exclusively derived from the liver.

Other sources include:

bone;
placenta;
intestine.

Therefore, an elevated ALP should be interpreted in clinical context and, where appropriate, with supporting information such as gamma-glutamyl transferase or other investigations.

A substantial hepatic ALP elevation can indicate cholestasis.

This matters because the classical Hy's Law concept concerns hepatocellular injury with bilirubin elevation, rather than a predominantly cholestatic process.

Total Bilirubin

Total bilirubin is an important marker in the evaluation of potential serious DILI.

Bilirubin can increase for multiple reasons, including:

hepatocellular dysfunction;
cholestasis;
biliary obstruction;
haemolysis;
inherited disorders;
other conditions.

Consequently, bilirubin elevation does not automatically establish liver failure or DILI.

The medical reviewer must establish the likely mechanism.

In a potential Hy's Law case, bilirubin elevation becomes particularly concerning when it occurs with significant hepatocellular injury and without evidence of a more likely alternative explanation.

Direct and Indirect Bilirubin

Total bilirubin is the traditional laboratory measure used in the Hy's Law framework.

Clinical evaluation may also benefit from understanding the direct and indirect components of bilirubin.

An isolated indirect hyperbilirubinaemia may have causes unrelated to hepatocellular injury, including haemolysis or inherited disorders such as Gilbert syndrome.

A predominantly direct hyperbilirubinaemia in the setting of hepatocellular injury may provide different clinical information.

The bilirubin result should therefore be interpreted in context rather than treated as a single binary variable.

Upper Limits of Normal

Hy's Law thresholds are generally expressed relative to the laboratory upper limit of normal.

For example:

ALT ≥3 × ULN

and

total bilirubin >2 × ULN

Using multiples of ULN helps account for differences between laboratories and reference ranges.

However, the ULN itself must be interpreted carefully.

The medical reviewer should establish:

the applicable laboratory reference range;
the relevant ULN at the time of the measurement;
whether reference ranges changed during the study;
whether local laboratory and central laboratory values differ;
whether the value was measured before or after treatment.

Incorrect use of ULN values can materially affect classification.

Baseline Liver Tests

The traditional thresholds are expressed using laboratory ULNs, but baseline status is clinically important.

A subject may enter a trial with:

elevated ALT;
elevated AST;
elevated ALP;
elevated bilirubin;
chronic viral hepatitis;
fatty liver disease;
cirrhosis;
other chronic liver abnormalities.

An increase during treatment must therefore be interpreted against the subject's baseline condition.

This becomes especially important in trials enrolling patients with chronic liver disease.

Consensus recommendations for trials involving chronic viral hepatitis, cirrhosis and related liver disease emphasise that applying standard laboratory thresholds without considering disease-related fluctuations can result in inappropriate attribution of liver-test abnormalities to the investigational drug. 4

Baseline Is Not Always a Single Number

A single screening laboratory result may not adequately describe a subject's baseline.

For subjects with fluctuating liver tests, investigators may need to consider multiple pretreatment values and the underlying disease trajectory.

This is especially relevant when:

the disease itself affects liver tests;
the subject has chronic hepatitis;
the subject has cirrhosis;
the subject has metabolic liver disease;
laboratory values fluctuate substantially.

A safety physician should therefore understand the subject's longitudinal laboratory history rather than looking only at the immediately preceding value.

INR and Hepatic Function

When evaluating serious liver injury, measures of hepatic synthetic function can provide important additional information.

Prothrombin time or INR may be particularly relevant.

A rising INR in the setting of acute hepatocellular injury can indicate impaired hepatic synthetic function.

This is clinically different from an isolated ALT elevation.

The safety physician should therefore review the available measures of liver function when assessing a potentially serious case.

Determining the Pattern of Liver Injury

Calculating the R-Value

The R-value is commonly used to characterise the biochemical pattern of liver injury.

It is calculated as:

R = (ALT / ALT ULN) / (ALP / ALP ULN)

The result helps classify injury as predominantly:

hepatocellular;
cholestatic;
mixed.

A commonly used classification is:

R-value	Pattern
>5	Hepatocellular
2 to 5	Mixed
<2	Cholestatic

These thresholds are widely used in DILI assessment.

The R-value is particularly useful because absolute ALT and ALP values alone do not fully describe the pattern.

For example, an ALT of 6× ULN may appear highly abnormal, but if ALP is also substantially elevated, the overall pattern may be mixed rather than predominantly hepatocellular.

Using the R-Value in DILI Assessment

The R-value provides another way of describing the relative contribution of ALT and ALP abnormalities.

R is calculated as:

R = (ALT / ALT ULN) / (ALP / ALP ULN)

A value above 5 is generally considered compatible with a hepatocellular pattern.

A value below 2 is generally considered cholestatic.

Values between 2 and 5 are generally considered mixed.

This approach is useful because it describes the relative magnitude of aminotransferase and ALP elevations rather than relying solely on an absolute ALP threshold.

However, the traditional FDA Hy's Law framework should not be silently replaced by the R-value.

The two approaches should be presented as related tools.

Why the Pattern Matters

The classical Hy's Law concept is based on hepatocellular injury accompanied by bilirubin elevation.

Therefore, correctly establishing the pattern of injury is critical.

A patient with:

ALT ≥3× ULN;
bilirubin >2× ULN;

does not automatically represent a Hy's Law case if the overall injury is predominantly cholestatic or if another explanation accounts for the bilirubin elevation.

The R-value can therefore support the medical evaluation by helping establish whether the biochemical pattern is consistent with hepatocellular injury.

Worked R-Value Example

Assume:

ALT = 450 U/L
ALT ULN = 50 U/L

ALP = 120 U/L
ALP ULN = 100 U/L

Then:

ALT / ULN = 9

ALP / ULN = 1.2

R = 9 / 1.2 = 7.5

The R-value is greater than 5, supporting a hepatocellular pattern.

If total bilirubin is also greater than 2× ULN, this patient would warrant evaluation for a potential Hy's Law pattern.

However, this calculation alone does not establish drug causality.

The medical reviewer must still assess the clinical context and competing causes.

Medical Evaluation of a Potential Hy's Law Case

Competing Causes

One of the most important elements of Hy's Law is the requirement to consider whether another cause better explains the findings.

Potential alternative explanations may include:

viral hepatitis;
autoimmune hepatitis;
biliary obstruction;
gallstone disease;
alcohol-related liver injury;
ischaemic hepatitis;
congestive hepatopathy;
pre-existing liver disease;
other hepatotoxic medications;
herbal or dietary supplements;
infections;
metabolic disease.

The extent of evaluation depends upon the clinical circumstances.

A patient should not be classified as a convincing Hy's Law case merely because laboratory thresholds are exceeded.

Temporal Relationship

Timing is an important component of causality assessment.

The medical reviewer should consider:

date of treatment initiation;
latency to liver-test abnormality;
dose changes;
treatment interruption;
treatment reintroduction;
time to peak laboratory abnormalities;
recovery after discontinuation.

A temporal relationship that is biologically plausible supports the assessment.

However, temporal association alone does not establish causality.

Dechallenge

Dechallenge refers to the course of the event after discontinuation or interruption of the suspected drug.

Improvement following withdrawal can support a drug-related hypothesis, although the interpretation depends on the natural history of the injury and other factors.

The reviewer should consider:

whether the suspected drug was discontinued;
whether liver tests improved;
the rate and magnitude of improvement;
whether other interventions occurred;
whether competing causes were treated.

Dechallenge should therefore be interpreted as part of the complete clinical picture.

Rechallenge

Rechallenge refers to recurrence or worsening of the event after the suspected drug is reintroduced.

A positive rechallenge can provide strong evidence supporting causality in appropriate circumstances.

However, deliberate rechallenge is generally not performed simply to establish a pharmacovigilance diagnosis because severe hepatic injury may recur and can be dangerous.

Where inadvertent or clinically necessary rechallenge occurs, the resulting information should be evaluated carefully.

Clinical Symptoms

Laboratory data should be interpreted alongside clinical findings.

Potentially relevant symptoms include:

jaundice;
dark urine;
pale stools;
nausea;
vomiting;
fatigue;
right upper quadrant discomfort;
pruritus;
anorexia;
confusion;
bleeding manifestations.

Symptoms do not establish causality, but they can materially change the assessment of severity and clinical significance.

Imaging and Other Investigations

Where clinically indicated, imaging and additional laboratory investigations can help evaluate alternative explanations.

Examples include:

abdominal ultrasound;
computed tomography;
magnetic resonance imaging;
viral serology;
autoimmune markers;
haemolysis investigations.

The appropriate work-up depends upon the clinical context.

The objective is not to perform every conceivable investigation in every subject.

The objective is to investigate plausible competing explanations sufficiently to support a medically defensible conclusion.

Drug-Specific Hepatic Signature

Some drugs produce characteristic patterns of liver injury.

A medical reviewer should therefore consider whether the observed case resembles the known hepatic safety profile of the suspected product.

Relevant characteristics may include:

latency;
biochemical pattern;
severity;
hypersensitivity;
autoimmune features;
recovery;
recurrence.

A novel product should also be assessed without assuming that the absence of a known signature excludes causality.

Causality Assessment

Hy's Law and causality assessment answer different questions.

Hy's Law asks whether the pattern is concerning for serious drug-induced hepatocellular injury.

Causality assessment asks whether the suspected medicinal product is responsible for the observed event.

Causality assessment may consider:

temporal association;
dechallenge;
rechallenge;
alternative causes;
known pharmacology;
previous evidence;
concomitant drugs.

Therefore, a potential Hy's Law case still requires a causality assessment.

RUCAM and Structured Causality Assessment

Structured causality methods such as RUCAM can be used in the evaluation of suspected DILI.

RUCAM considers multiple components, including:

time to onset;
course after drug withdrawal;
risk factors;
concomitant drugs;
exclusion of non-drug causes;
previous information on hepatotoxicity;
response to rechallenge.

Hy's Law should not be confused with RUCAM.

The former identifies a concerning pattern associated with serious DILI risk.

The latter is a structured approach to assessing causality.

Both may contribute to a comprehensive medical evaluation.

Hy's Law in Clinical Development

Why Clinical Development Is Challenging

Hy's Law has particular importance during clinical development.

The reason is statistical.

Severe DILI is sufficiently rare that a clinical programme may contain no cases of acute liver failure even when a drug ultimately carries a clinically important hepatotoxicity risk.

A programme may nevertheless identify subjects with:

substantial ALT elevations;
bilirubin elevations;
compatible clinical findings;
a temporal relationship to treatment.

These cases can provide an early warning signal.

FDA's guidance on premarketing clinical evaluation of DILI specifically addresses how laboratory measurements can be obtained and evaluated to identify signals of potential severe liver injury during drug development. [1]

Clinical-Trial Liver-Safety Data Review

In clinical trials, liver-safety evaluation is usually performed at both the individual-subject and population levels.

Individual cases may be medically reviewed.

At the population level, investigators may examine:

maximum ALT;
maximum AST;
maximum ALP;
maximum bilirubin;
treatment-emergent changes;
baseline-adjusted changes;
time to onset;
time to recovery;
concurrent bilirubin elevations;
treatment group differences.

This allows the safety team to determine whether there is an imbalance or pattern that warrants further investigation.

Clinical-Trial Monitoring and Actions

A potential Hy's Law case may trigger predefined clinical-trial actions.

Depending upon the protocol and circumstances, these may include:

repeat liver testing;
increased monitoring;
temporary interruption;
permanent discontinuation;
specialist referral;
expedited safety review.

The exact action should be defined by the clinical-development programme and medical governance.

Hy's Law should not be interpreted as an automatic universal stopping rule.

Regulatory Decision-Making

Potential Hy's Law cases may influence regulatory assessment of a drug's benefit-risk profile.

During development, regulators may consider:

number of potential cases;
quality of case adjudication;
exposure;
dose relationship;
temporal pattern;
competing causes;
background incidence;
reversibility;
other liver-safety findings.

The presence of convincing Hy's Law cases can therefore trigger additional investigation and may influence the overall development and regulatory strategy.

It is not, however, an automatic decision rule for approval or rejection.

Hy's Law in Post-Marketing Pharmacovigilance

Post-Marketing Case Evaluation

Hy's Law is most strongly associated with clinical-trial liver-safety evaluation, but the underlying medical reasoning remains relevant after approval.

In post-marketing pharmacovigilance, safety teams may encounter potential DILI through:

spontaneous adverse-event reports;
solicited reports;
literature;
patient support programmes;
regulatory reports;
safety databases;
epidemiological studies;
post-authorisation studies.

A post-marketing case may contain incomplete laboratory information.

Therefore, the classical Hy's Law thresholds cannot always be applied directly to an individual spontaneous report.

The medical reviewer should distinguish between:

a case that clearly satisfies the relevant biochemical pattern;
a case suggestive of possible DILI but lacking sufficient laboratory information;
a clinically severe hepatic event without the laboratory data needed to assess Hy's Law;
a case where an alternative cause is more likely.

This distinction prevents inappropriate classification of every hepatic adverse event as a Hy's Law case.

Aggregate Safety Assessment

Post-marketing safety assessment may include review of serious hepatic cases across:

spontaneous reports;
clinical studies;
literature;
registries;
epidemiological studies;
post-authorisation studies.

The safety team may look for:

repeated hepatocellular patterns;
bilirubin elevations;
acute liver failure;
transplantation;
deaths;
positive dechallenge;
positive rechallenge;
consistent latency;
common risk factors.

Hy's Law concepts can help organise the clinical evaluation, but the evidence must be interpreted in the context of the data source.

Signal Detection and Evaluation

At the population level, a convincing Hy's Law pattern can contribute to a drug-safety signal.

The signal may then require:

case-level adjudication;
additional laboratory analyses;
exposure-adjusted analysis;
comparison with control;
review of concomitant medications;
review of clinical outcomes;
regulatory discussion.

The strength of the signal depends on the quality and consistency of the evidence.

eDISH and Population-Level Liver Safety

What Is eDISH?

Evaluation of Drug-Induced Serious Hepatotoxicity, commonly abbreviated eDISH, is a graphical approach used to examine liver-test data in clinical trials.

The approach plots measures such as:

peak ALT relative to ULN;
peak total bilirubin relative to ULN.

The resulting distribution allows investigators to identify subjects whose laboratory results fall into regions of particular concern.

eDISH is therefore complementary to Hy's Law.

Hy's Law provides a conceptual and clinical framework.

eDISH provides a visual population-level tool that can help identify subjects requiring further review.

Published research has used eDISH to retrospectively evaluate Hy's Law cases and liver-safety patterns. [4]

How eDISH Works

eDISH stands for evaluation of Drug-Induced Serious Hepatotoxicity.

It is commonly used to visualise the relationship between maximum or relevant ALT and total bilirubin values during clinical development.

The plot helps identify subjects in regions associated with potential Hy's Law concern.

A typical eDISH representation divides the plot into quadrants using:

ALT at 3× ULN;
total bilirubin at 2× ULN.

The upper-right region contains subjects with both substantial ALT and bilirubin elevations.

These subjects warrant particular attention.

The plot is a screening and visualisation tool, not an automated causality algorithm.

Why eDISH Is Useful

Large clinical-trial datasets can contain thousands or millions of laboratory measurements.

A table of laboratory values can make it difficult to see whether a small number of subjects have an important combined pattern.

eDISH compresses this information into a visual representation.

It can help investigators identify:

subjects requiring review;
treatment-group patterns;
clusters;
outliers;
changes over time.

Published work has demonstrated the practical use of eDISH for identifying potential Hy's Law cases in clinical datasets. 6

Why eDISH Does Not Determine Causality

An eDISH plot cannot determine whether the investigational product caused a liver injury.

A subject in the upper-right region may have:

viral hepatitis;
another hepatotoxic medication;
biliary disease;
pre-existing liver disease;
laboratory error;
another explanation.

The plot therefore answers:

"Which subjects deserve attention?"

It does not answer:

"Did the drug cause this event?"

That second question requires medical review.

eDISH and Medical Review

A plot can identify a potentially important region of laboratory space.

It cannot independently determine:

causality;
competing diagnoses;
clinical symptoms;
treatment relationship;
whether bilirubin elevation is hepatic;
whether ALP is hepatic;
whether a patient has an alternative explanation.

Therefore, an eDISH outlier should trigger medical review rather than automatically being labelled a Hy's Law case.

Worked Examples

Example 1: Classic Potential Hy's Law Pattern

Consider a subject receiving an investigational medicine.

Baseline:

ALT = 32 U/L
ALT ULN = 40 U/L

ALP = 90 U/L
ALP ULN = 120 U/L

Total bilirubin = 0.8 mg/dL
Bilirubin ULN = 1.2 mg/dL

At week 8:

ALT = 360 U/L
ALP = 150 U/L
Total bilirubin = 3.0 mg/dL

The relative values are:

ALT = 9× ULN

ALP = 1.25× ULN

Bilirubin = 2.5× ULN

The pattern is predominantly hepatocellular.

The laboratory findings therefore warrant evaluation for a potential Hy's Law case.

But the evaluation does not stop here.

The reviewer should investigate:

viral hepatitis;
concomitant medicines;
supplements;
alcohol exposure;
biliary disease;
underlying liver disease;
symptoms;
treatment timing;
subsequent laboratory results;
dechallenge.

Only after that assessment can the case be characterised appropriately.

Example 2: Alternative or Cholestatic Explanation

Consider another subject with:

ALT = 400 U/L
ALP = 600 U/L
Total bilirubin = 4.0 mg/dL

Suppose:

ALT ULN = 40 U/L

ALP ULN = 120 U/L

The relative values are:

ALT = 10× ULN

ALP = 5× ULN

R = 10 / 5 = 2

This is at the boundary of the mixed pattern rather than a clearly hepatocellular pattern.

If imaging simultaneously demonstrates biliary obstruction, the bilirubin elevation may have a compelling alternative explanation.

This case should therefore not be labelled a convincing Hy's Law case solely because the ALT and bilirubin thresholds are exceeded.

The clinical explanation matters.

Example 3: Bilirubin Elevation Without Significant Hepatocellular Injury

Consider a subject with:

ALT = 45 U/L
ALT ULN = 40 U/L

ALP = 110 U/L
ALP ULN = 120 U/L

Total bilirubin = 3.5 mg/dL
Bilirubin ULN = 1.2 mg/dL

The bilirubin is substantially elevated, but the aminotransferase elevation is minimal.

The pattern does not satisfy the traditional biochemical trigger for a Hy's Law case.

The reviewer should investigate why bilirubin is elevated.

Potential explanations could include:

Gilbert syndrome;
haemolysis;
biliary disease;
medication-related bilirubin effects;
other causes.

This illustrates why bilirubin should never be interpreted independently.

Example 4: Competing Hepatotoxic Drug

Consider a subject receiving Drug A who develops:

ALT = 500 U/L
ALP = 130 U/L
bilirubin = 3.0 mg/dL

Drug A is potentially hepatotoxic.

However, the subject also started Drug B two weeks earlier, and Drug B has a well-established association with acute hepatocellular DILI.

The correct conclusion is not automatically:

Drug A caused a Hy's Law case.

Instead, the case requires comparative causality assessment.

The reviewer should examine:

latency for both drugs;
known hepatotoxic signatures;
dechallenge;
rechallenge;
previous exposure;
clinical course;
competing causes.

The Hy's Law pattern can indicate serious liver injury without uniquely identifying the responsible drug.

Limitations of Hy's Law

Important Limitations

Hy's Law has important limitations.

First, it is not a sensitive predictor of acute liver failure in individual patients.

A retrospective study of patients with DILI found that Hy's Law had high specificity and negative predictive value but substantially lower sensitivity and positive predictive value for acute liver failure. [3]

Second, the concept focuses primarily on hepatocellular injury with bilirubin elevation.

Serious outcomes can also occur with other patterns of liver injury.

Third, laboratory abnormalities may have causes unrelated to the drug.

Fourth, the classical thresholds may not capture every clinically important presentation.

Fifth, the concept was developed primarily for evaluating drug-related risk at the population and drug-development level rather than functioning as a bedside prognostic score for an individual patient.

These limitations should be understood whenever Hy's Law is applied.

The Historical 10% Concept

A frequent misunderstanding is that meeting Hy's Law means that an individual patient has a 10% probability of dying or requiring transplantation.

That interpretation is incorrect.

The often-cited approximate 10% figure relates to the historical observation that, among drugs capable of causing serious hepatocellular injury with jaundice, the occurrence of several convincing cases may indicate a clinically important risk of severe outcome at the population level.

It should not be converted directly into an individual-patient probability.

The actual risk depends upon:

the drug;
exposure;
patient population;
clinical context;
case definition;
competing causes;
duration of follow-up;
ascertainment.

This distinction is essential for accurate communication.

The Three-Cases Concept

Traditional drug-development discussions sometimes refer to the occurrence of approximately three convincing Hy's Law cases as a particularly concerning signal.

This concept should be interpreted cautiously.

The purpose is not that exactly three cases establish causality or regulatory action.

Rather, multiple convincing cases occurring in a sufficiently exposed clinical-development programme can provide evidence that a drug may have a meaningful risk of serious hepatotoxicity.

The number of cases must be interpreted in relation to:

total exposure;
background incidence;
competing causes;
clinical trial population;
timing;
dose;
concomitant medications;
quality of case evaluation.

Therefore, the "three cases" concept should never be used as an automatic regulatory rule.

The Central Limitation

Hy's Law is valuable because it converts a relatively uncommon and difficult-to-observe clinical outcome into an earlier safety signal.

Its strength comes from combining:

hepatocellular injury;
bilirubin elevation;
exclusion of competing explanations;
population-level context.

Its limitation is equally important:

the pattern is a warning signal, not a diagnosis and not an individual prognosis.

The medical reviewer must therefore preserve the distinction between:

laboratory abnormality;
liver injury;
DILI;
potential Hy's Law;
convincing Hy's Law case;
acute liver failure.

These are related but different clinical concepts.

Practical Medical Review Framework

Step-by-Step Medical Review

A structured medical review may proceed through the following sequence:

Confirm the laboratory values.
Confirm the applicable ULNs.
Determine the pattern of liver injury.
Assess ALT and AST elevations.
Assess ALP.
Assess total and direct bilirubin where available.
Determine the time relationship to drug exposure.
Review concomitant medications.
Review relevant medical history.
Evaluate competing causes.
Review symptoms and clinical findings.
Assess dechallenge.
Assess any rechallenge.
Determine whether the overall case is compatible with DILI.
Determine whether the case meets the applicable Hy's Law definition.
Document the medical rationale.

This approach is more reliable than applying laboratory thresholds mechanically.

Medical Documentation

For an important potential Hy's Law case, the medical assessment should make the reasoning transparent.

The assessment should address, as appropriate:

relevant laboratory values;
ULNs;
R-value;
pattern of injury;
bilirubin;
clinical symptoms;
treatment dates;
latency;
dechallenge;
rechallenge;
concomitant drugs;
relevant medical history;
alternative causes;
imaging or specialist assessment;
overall causality;
seriousness;
outcome.

The objective is to allow another medically qualified reviewer to understand how the conclusion was reached.

Pharmacovigilance and Inspection Considerations

Inspection Perspective

During an inspection, an organisation should be able to demonstrate that potential serious liver-safety signals were identified, reviewed and appropriately followed.

An inspector may reasonably expect to understand:

how liver-test abnormalities were identified;
how potential Hy's Law cases were defined;
who performed medical review;
how competing causes were evaluated;
how cases were documented;
how the aggregate signal was assessed;
how relevant findings affected risk management.

The important evidence is therefore not simply the existence of an eDISH plot.

The organisation should be able to demonstrate the decision-making process behind the plot and the resulting medical conclusions.

Common Documentation Weaknesses

Potential weaknesses include:

unexplained classification of a case as Hy's Law;
missing laboratory reference ranges;
inadequate baseline information;
failure to document competing causes;
no explanation of why ALP was considered acceptable;
unclear causality reasoning;
no documented medical review of eDISH outliers;
failure to reconcile clinical narratives with laboratory data;
inconsistent classification across cases.

Good documentation should allow an independent reviewer to reconstruct the medical reasoning.

Common Mistakes in Hy's Law Assessment

Common errors in Hy's Law assessment include:

treating ALT ≥3× ULN and bilirubin >2× ULN as sufficient by themselves;
ignoring the injury pattern;
failing to evaluate ALP;
failing to consider alternative causes;
treating bilirubin elevation as proof of liver failure;
assuming AST is equivalent to ALT for every purpose;
ignoring baseline laboratory values;
ignoring the timing of treatment;
confusing Hy's Law with causality assessment;
treating the three-case concept as an automatic regulatory threshold;
interpreting the 10% historical figure as an individual risk;
assuming absence of a Hy's Law pattern excludes serious DILI;
treating eDISH outliers as confirmed Hy's Law cases;
failing to document the medical reasoning.

What an Experienced Safety Physician Looks For

An experienced safety physician does not start with the question:

"Does this patient meet Hy's Law?"

The first question is:

"What is the clinical pattern of liver injury, and what is the most plausible explanation for it?"

The reviewer then considers:

Is the injury genuinely hepatocellular?
Is the bilirubin elevation clinically meaningful?
Is there evidence of cholestasis?
What happened before treatment?
What happened after treatment?
What other medicines were present?
Is there a competing diagnosis?
Is the laboratory pattern reproducible?
Is there a biologically plausible latency?
Did the patient recover after withdrawal?
Is there evidence across other patients?

Only after these questions have been considered should the reviewer decide whether the case represents a convincing potential Hy's Law case.

This approach prevents numerical thresholds from replacing clinical judgement.

Key Takeaways

Hy's Law is an important drug-safety concept for identifying a pattern of hepatocellular injury accompanied by bilirubin elevation that may indicate a risk of serious drug-induced liver injury.

The commonly used biochemical criteria include significant ALT or AST elevation and bilirubin elevation, together with an absence of a better alternative explanation and without a predominantly cholestatic pattern.

The criteria should never be applied mechanically.

A robust assessment requires:

confirmation of laboratory values;
assessment of the injury pattern;
consideration of ALT, AST, ALP and bilirubin;
use of the R-value where appropriate;
assessment of temporal relationship;
review of concomitant medicines;
evaluation of competing causes;
assessment of dechallenge and rechallenge;
clinical evaluation of severity and outcome.

Hy's Law is particularly important in clinical development because severe idiosyncratic DILI is rare and clinical trials may not directly observe enough cases of liver failure to estimate risk reliably.

It is therefore best understood as a signal concept for potential serious hepatotoxicity, not as an individual-patient prognostic test or a substitute for medical causality assessment.

eDISH and other population-level approaches can complement the evaluation by identifying patterns in clinical-trial liver-test data, but they do not replace individual medical review.

The most important principle is simple:

Hy's Law is a structured warning signal that requires clinical interpretation, not a laboratory threshold that produces an automatic diagnosis.

References

U.S. Food and Drug Administration. Drug-Induced Liver Injury: Premarketing Clinical Evaluation. Guidance for Industry. July 2009.
Temple R. Hy's law: predicting serious hepatotoxicity. Pharmacoepidemiology and Drug Safety. 2006;15(4):241-243. doi:10.1002/pds.1211.
Lo Re V III, Haynes K, Forde KA, et al. Risk of acute liver failure in patients with drug-induced liver injury: evaluation of Hy's law and a new prognostic model. Clinical Gastroenterology and Hepatology. 2015;13(13):2360-2368.e5. doi:10.1016/j.cgh.2015.06.020.
Shen X, Yuan Z, Mei J, et al. Anti-tuberculosis drug-induced liver injury in Shanghai: validation of Hy's Law. Drug Safety. 2013. doi:10.1007/s40264-013-0119-6.
Robles-Diaz M, Lucena MI, Kaplowitz N, et al. Use of Hy's law and a new composite algorithm to predict acute liver failure in patients with drug-induced liver injury. Gastroenterology. 2014;147(1):109-118.e5. doi:10.1053/j.gastro.2014.03.050.
Regev A, Björnsson ES. Drug-induced liver injury: morbidity, mortality, and Hy's law. Gastroenterology. 2014;147(1):20-24. doi:10.1053/j.gastro.2014.05.027.
Senior JR. How can "Hy's law" help the clinician? Pharmacoepidemiology and Drug Safety. 2006;15(4):235-239. doi:10.1002/pds.1209.