Lab Literacy / The Safety Lab Gate: What Trials Check Before Anyone Starts
9 min readEvery peptide trial has two phases the public never sees. The first is the one you expect: researchers give a compound, measure what happens, and publish the results. The second is less visible and arguably more important: before anyone receives anything, every participant goes through a battery of blood tests, and some of them don't make it past that screen.
This isn't bureaucracy. These exclusion thresholds exist because a drug that's safe in a healthy volunteer can be dangerous in someone whose kidneys are already struggling, whose liver is inflamed, or whose thyroid is quietly out of range. The gate isn't about the drug. It's about the person the drug enters.
The thresholds repeat across trials with striking consistency. Here is what they are, where they come from, and why each one matters.
The most common exclusion across all peptide trial types is an estimated glomerular filtration rate below 30 mL/min/1.73 m².
eGFR is the standard measure of how well your kidneys filter waste from blood. The full staging runs from stage 1 (90 or above, normal) through stage 5 (below 15, end-stage). Stage 4 — moderate-to-severe impairment, an eGFR of 15–29 — is where the SURMOUNT trials for tirzepatide drew their line.
Why 30 and not some other number? The answer is pharmacokinetic. Many peptides and their metabolites are cleared at least partially through the kidneys. When filtration drops below a certain point, drug exposure accumulates faster than the body can eliminate it. A dose calibrated for normal clearance becomes an escalating dose in someone with impaired clearance. That's not a theoretical concern — it's the mechanism behind dose-adjustment guidelines for dozens of approved drugs, and it's the reason trialists gate on it before enrollment rather than discovering the problem during the study.
The eGFR threshold is sometimes tiered. The SURMOUNT protocol for tirzepatide used a stricter calcitonin cutoff (see below) for participants with eGFR below 60, acknowledging that even moderate kidney impairment changes the risk calculus for certain adverse effects.
The second most consistent gate is a liver enzyme called alanine aminotransferase, or ALT. Trials routinely exclude anyone whose ALT exceeds three times the upper limit of normal (3× ULN).
ALT is an enzyme that leaks into the blood when liver cells are damaged. The normal range in most labs runs roughly 4–56 U/L, so 3× ULN sits around 168 U/L. An ALT at that level signals active and significant liver injury — not minor inflammation, but something that is visibly stressing hepatocytes.
The logic is straightforward. If the liver is already injured, asking it to metabolise an investigational compound adds load to a system that's already signalling distress. The concern isn't only that the drug might worsen the injury (though that's possible). It's also that liver injury changes drug metabolism in unpredictable ways — a compound that's safe at a given dose in a healthy liver can reach higher-than-expected blood levels in an injured one, because the metabolic pathways that would normally break it down are compromised.
The SURMOUNT protocol added a second liver gate: bilirubin above 1.2× ULN. Elevated bilirubin alongside elevated ALT suggests a pattern of liver dysfunction that is more systemic than a single enzyme can capture. Some trials also track AST (aspartate aminotransferase) and use the ratio of AST to ALT — a ratio above 2:1 suggests the source may be muscle or alcohol rather than the liver itself, which changes the interpretation.
Growth hormone-axis peptides and GLP-1 agonists both interact with the endocrine system in ways that make thyroid function a necessary baseline. The SURMOUNT trials excluded anyone with a thyroid-stimulating hormone (TSH) outside the range of 0.4–6.0 mIU/L.
The standard clinical reference range for TSH is narrower — roughly 0.4–4.5 mIU/L. Trials widen the upper bound to 6.0 to capture people with mild subclinical hypothyroidism who might otherwise be missed, while still excluding those with more significant thyroid dysfunction.
Why does it matter? Growth hormone directly affects the conversion of thyroxine (T4) into its active form, triiodothyronine (T3). Someone whose thyroid is already underperforming may not handle the additional metabolic demand. On the other side, undiagnosed hyperthyroidism — a TSH below 0.4 — brings its own set of risks that could confound trial outcomes. The gate exists to keep the endocrine playing field level.
One exclusion threshold is specific to the GLP-1 agonist class, and it has a specific reason.
Calcitonin is a hormone produced by the C-cells of the thyroid. In rodents, GLP-1 receptor agonists caused C-cell tumours — a finding that drove the FDA to require a Risk Evaluation and Mitigation Strategy (REMS) for this drug class. Human relevance is debated (rodent C-cells respond differently to GLP-1 than human C-cells do), but the precautionary principle applies: trials and prescribing information both flag calcitonin as a surveillance marker.
The SURMOUNT protocol set the exclusion at a calcitonin level of 20 ng/L or higher in participants with normal kidney function (eGFR ≥60), and 35 ng/L or higher in those with reduced kidney function (eGFR <60). The lower threshold for people with impaired kidneys reflects the fact that calcitonin is partly cleared renally — reduced clearance can produce mildly elevated levels that don't necessarily signal a thyroid problem.
This is the only exclusion threshold in the set that exists primarily because of a preclinical animal finding rather than a known human toxicity. It's also the one most often discussed — and sometimes overstated — in patient communities. The nuance matters: the rodent signal is real, the human relevance is uncertain, and the threshold exists as a guard rail, not as evidence of confirmed risk.
The SURMOUNT trials excluded anyone with a blood pressure of 160/100 mmHg or higher. The tesamorelin trials excluded people with untreated hypertension.
Blood pressure is the least specific of the safety gates, but that's precisely why it matters. Severely elevated blood pressure signals cardiovascular risk that could be compounded by an investigational compound — particularly one that affects metabolism, body composition, or fluid balance. Rather than trying to predict which compound might interact with which aspect of cardiovascular function, trialists draw a line and keep uncontrolled hypertension out.
| Marker | Exclusion Threshold | Primary Concern | Trial Source |
|---|---|---|---|
| eGFR | <30 mL/min/1.73 m² | Drug accumulation from impaired clearance | SURMOUNT |
| ALT | >3× ULN (~168 U/L) | Liver injury compromises drug metabolism | SURMOUNT |
| Bilirubin | >1.2× ULN | Systemic liver dysfunction | SURMOUNT |
| TSH | <0.4 or >6.0 mIU/L | Endocrine instability under peptide intervention | SURMOUNT |
| Calcitonin | ≥20 ng/L (eGFR≥60); ≥35 ng/L (eGFR<60) | Medullary thyroid cancer surveillance (rodent signal) | SURMOUNT |
| Blood pressure | ≥160/100 mmHg | Uncontrolled cardiovascular risk | SURMOUNT |
Some trial protocols add markers that are compound-specific rather than class-wide. GLP-1 trials monitor lipase and amylase at baseline because of the known pancreatitis signal in this drug class. Growth hormone trials check prolactin because some secretagogues have been observed to elevate it. Trials involving tesamorelin required CD4 counts above 100 and viral load below 10,000, reflecting the drug's approval in HIV-associated lipodystrophy — an exclusion that says more about the approved population than about the compound's pharmacology.
The core set — kidneys, liver, thyroid, calcitonin, blood pressure — is what repeats. When you see these same five gates across trials for completely different peptides targeting completely different pathways, that's not coincidence. It's a consensus about the minimum safety infrastructure required before exposing a human being to an investigational compound.
When a peptide trial reports its results, the safety gate has already done its work. The participants who received the compound all passed these thresholds. That means the reported safety profile applies to a pre-selected population whose kidneys, liver, thyroid, and blood pressure were within defined ranges at the start. It does not describe what happens in someone who would have been excluded.
This is the single most important thing to understand when translating trial safety data to any broader context. A trial that reports "no serious adverse events related to liver function" is reporting on a population whose ALT was below 168 U/L at baseline. That finding is genuine and meaningful — but it's bounded by the gate. Someone with an ALT of 200 who reads that headline and concludes the compound is liver-safe for them is extrapolating past the data.
The gate also explains why some trials report very clean safety profiles. It's not that the compound is harmless. It's that the trial already removed the people most likely to be harmed.
This article is for educational purposes and is not medical advice. It does not recommend any compound, test, or protocol. Decisions about medical testing and treatment belong with a qualified clinician who knows your history.
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