Peptide & Experimental
Sobetirome (GC-1)
Selective thyroid hormone receptor beta agonist, research substance · Sobetirome, GC-1, QRX-431
Sobetirome, usually called GC-1 in the scene, is a synthetic derivative of thyroid hormone that preferentially targets the receptor in the liver. It was developed as a cholesterol-lowering drug meant to spare the heart, and in animal experiments it did exactly that, including weight loss. Phase 1 studies were conducted in humans, but their data were never fully published. The principle of action has since proven itself with the related resmetirom, which is approved for fatty liver inflammation; for fat loss with sobetirome there is no human study.
What sobetirome is
Sobetirome is a so-called thyromimetic: a molecule modeled on the thyroid hormone T3 but intended to trigger only part of its effects. It comes from the laboratory of the chemist Thomas Scanlan and was taken into clinical testing as QRX-431. Development as a cholesterol-lowering drug was not continued after phase 1.
The compound has remained of interest for two reasons. First, it is being discussed for rare diseases, such as X-linked adrenoleukodystrophy. Second, the underlying principle, targeted activation of the liver receptor, has been confirmed in humans with resmetirom. On the gray market, GC-1 is offered as a research substance for fat loss; unlike the thyroid hormone liothyronine, it is not approved as a medicine anywhere.
How it works
Thyroid hormone acts via two receptor types. TR-alpha is found mainly in the heart, muscle and bone and is responsible for a racing heart and bone loss when there is too much hormone. TR-beta predominates in the liver, where it controls cholesterol breakdown and fat burning. Sobetirome binds preferentially to TR-beta and additionally accumulates in the liver.
In rats, GC-1 lowered cholesterol about 30 times more effectively than it raised the pulse, and it increased the basal metabolic rate without the heart following suit as it does under T3. In cynomolgus monkeys, cholesterol and lipoprotein(a) fell after 7 days, and body weight by about 4 percent, without a rise in pulse. These animal data are the basis for its reputation as a fat burner. In humans, the related resmetirom has shown in a large phase 3 trial that liver-directed TR-beta activation can reduce fatty liver inflammation and LDL cholesterol.
What is well supported
- The receptor selectivity. The preference for TR-beta over TR-alpha and the accumulation in the liver are well described in the laboratory and in several animal models.
- Lowering of cholesterol and weight in animals without strain on the heart. In monkeys, cholesterol, lipoprotein(a) and weight fell within one week; the pulse remained unchanged.
- Well tolerated in humans, as far as reported. Reviews describe the phase 1 studies as successful and without apparent harmful side effects. The original data, however, have not been published.
- The principle holds up in humans. Resmetirom, also a liver-selective TR-beta agonist, met both primary endpoints in phase 3 and has held conditional approval in the EU since August 2025.
What the studies show
Grover 2004: rats and monkeys
The study (Endocrinology) compared GC-1 with the natural hormone T3. In rats fed a high-cholesterol diet, GC-1 lowered cholesterol at a dose roughly 30 times lower than was needed to raise the pulse; T3 did not show this gap. The basal metabolic rate also rose, with about 10-fold selectivity relative to the pulse. In cynomolgus monkeys, T3 and GC-1 lowered cholesterol, lipoprotein(a) and body weight by about 4 percent over 7 days; a racing heart occurred only under T3.
The phase 1 studies in humans
Scanlan 2010 and Lammel Lindemann 2016 describe that sobetirome went through phase 1 studies in humans, with “excellent results” and without obvious harmful side effects. We have not found a complete publication of the study results in a journal; participant numbers, doses and measurements therefore cannot be verified. The authors of the 2016 review consider further studies worthwhile, among other things for rare diseases and for short-term, controlled weight loss.
Resmetirom: the proof of principle
Harrison 2024 (NEJM) randomized 966 patients with MASH and fibrosis. After 52 weeks, MASH resolved in 25.9 and 29.9 percent compared with 9.7 percent under placebo, and fibrosis improved in 24.2 and 25.9 compared with 14.2 percent. LDL cholesterol fell by 13.6 and 16.3 percent. Resmetirom is a different molecule; the results do not apply to sobetirome, but they show that the approach can work in humans.
Eprotirome: the warning from the class
Sjouke 2014 (Lancet Diabetes Endocrinol) tested the related compound eprotirome in 236 people with familial hypercholesterolemia. LDL fell, but liver values and bilirubin rose, and free T4 fell by 19 to 27 percent. The study was stopped early after cartilage damage had occurred in dogs in another study.
Where the data stop
- No verifiable human data. The phase 1 results have not been published. How sobetirome affects cholesterol, weight, heart and thyroid in humans cannot be verified.
- Fat loss. The weight data come from one-week animal experiments. There is no study on body composition in humans.
- Long-term safety. The experience with eprotirome shows that the liver and cartilage can be affected. Whether this applies to sobetirome has never been investigated.
- Products from the gray market. What is sold as a research substance has not been independently tested for content or purity.
Status, approval and legal
Sobetirome is not approved as a medicine anywhere in the world. A registered study in X-linked adrenoleukodystrophy was withdrawn before anyone took part. In Germany, sobetirome is not listed in Annex 1 of the German Prescription Medicines Ordinance (Arzneimittelverschreibungsverordnung), unlike liothyronine and resmetirom. A product intended for use in humans would nevertheless require approval.
In sports, sobetirome, as a substance not approved anywhere, falls under class S0 of the WADA Prohibited List 2026 and is banned at all times. We do not give dosages.
Safety
There are no published safety data in humans. The reviews mention no conspicuous side effects from phase 1, but give no figures.
What is known from the substance class: in humans, eprotirome raised liver values and lowered free T4, that is, the body’s own thyroid hormone production; in dogs, cartilage damage occurred. The selectivity for the liver receptor is relative, not absolute, so effects on the heart, bones and thyroid axis cannot be ruled out with uncontrolled use. Anyone who notices symptoms such as a racing heart or restlessness should have their thyroid and liver values checked by a doctor.
BK-Score Tested in humans, results unpublished
| Human evidence | 1 | |
|---|---|---|
| Mechanism | 6 | |
| Safety data | 1 | |
| Hype gap | 2 | |
| Track record of use | 2 |
Evidence 1, because sobetirome was tested in phase 1 studies in humans, but their results never appeared in full in a journal; reviews describe them only in summary as well tolerated and cholesterol-lowering (Scanlan 2010; Lammel Lindemann 2016), and for fat loss, for which it is offered on the gray market, there is not a single human study. Mechanism 6, because the preference for the liver receptor TR-beta is well described in the laboratory and in animal experiments, with cholesterol and weight reduction without a rise in pulse in monkeys (Grover 2004), and because the principle has been confirmed in humans with resmetirom (Harrison 2024); confirmation in humans is lacking for sobetirome itself. Safety 1, because no published safety data in humans exist and the class carries warning signals: the related compound eprotirome raised liver values and was stopped after cartilage damage in dogs (Sjouke 2014). Hype 2, because GC-1 is traded as a fat burner, although the weight data come exclusively from animal experiments. Use 2, because the substance appears only briefly in early studies and otherwise on the gray market. Direction open: there are no human data for the advertised effect.
The score rates the state of knowledge, not the substance. “Safety data 9” means well studied – not harmless.
Subjective assessment by Biohacking Kompakt based on published scoring rules – not a scientific rating and not a medical recommendation. Rules and all ratings (German)
Frequently asked questions about sobetirome (GC-1)
What is sobetirome (GC-1)?
Sobetirome is a synthetic derivative of thyroid hormone that preferentially activates the TR-beta receptor in the liver. It was developed as a cholesterol-lowering drug, went through phase 1 studies and is not approved anywhere.
Does GC-1 help with weight loss?
That has not been studied in humans. In monkeys, weight fell by about 4 percent in one week; in rats, the basal metabolic rate rose. There are no human studies on fat loss.
What is the difference from resmetirom?
Both are liver-selective TR-beta agonists. Resmetirom was tested in a phase 3 trial with 966 patients and has held conditional approval in the EU since August 2025 for MASH with fibrosis. Sobetirome did not get beyond phase 1, and those data are unpublished.
Is sobetirome the same as T3?
No. T3 acts on both receptor types and therefore also on the heart and bones. Sobetirome prefers the liver receptor. In animals it lowered cholesterol without a racing heart; whether this is the case in humans has not been shown in published data.
Is sobetirome legal in Germany?
It is not an approved medicine and is not listed in the German Prescription Medicines Ordinance. In sports, as a substance not approved anywhere, it is banned at all times under the WADA List 2026 (S0).
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Sources
- Scanlan TS, Heart Fail Rev 2010 – Sobetirome: development history from laboratory to clinic
- Lammel Lindemann J, Webb P, Expert Opin Ther Targets 2016 – Sobetirome: past, present, open questions
- Grover GJ et al., Endocrinology 2004 – GC-1 in rats and monkeys: metabolism, cholesterol, pulse
- Harrison SA et al., N Engl J Med 2024 – MAESTRO-NASH, resmetirom phase 3
- EMA – Rezdiffra (resmetirom), European public assessment report
- Sjouke B et al., Lancet Diabetes Endocrinol 2014 – eprotirome, AKKA trial, termination
- ClinicalTrials.gov NCT01787578 – sobetirome in X-ALD, withdrawn
- German Prescription Medicines Ordinance (Arzneimittelverschreibungsverordnung), Annex 1
- WADA – Prohibited List 2026, S0 Non-approved substances
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Information only, not medical advice and not a usage or dosage recommendation. Prescription-only and unapproved substances belong in the hands of a physician. Last updated: 2026-10-09.