Thyroid hormones (T3 / T4)

Liothyronine / Cytomel / T3, Levothyroxine / Synthroid / T4, Sobetirome / GC-1 · Thyroid hormone (metabolic)

Research reference only. Pepdex does not sell, supply or track this compound, and nothing here is guidance to use it — this page exists so the evidence and the risks are visible.

What it is

Prescription thyroid hormones — levothyroxine (T4) and liothyronine (T3) — taken without thyroid disease in an attempt to raise metabolic rate during diets. Unapproved thyromimetics like GC-1 (sobetirome) circulate under the same umbrella.

How it works

T4 (levothyroxine) and T3 (liothyronine) are the hormones that set your metabolic rate. T4 is largely a storage form the body converts to the active T3 as needed; taking T3 directly bypasses that regulation. Raising thyroid hormone above your own level increases energy expenditure, which is why it is misused for fat loss — but the body does not distinguish between fat and muscle when it accelerates metabolism, and a well-documented share of weight lost this way is lean tissue. Crucially, taking external thyroid hormone suppresses your own thyroid axis, so this is not a supplement you simply add.

What human evidence shows

The metabolic effect is textbook endocrinology. Historical studies in euthyroid people exist, but they do not establish an acceptable benefit-risk profile for weight loss — hyperthyroid states burn muscle alongside fat.

The studies, one by one

Levothyroxine is one of the most-prescribed drugs in the world with unambiguous evidence for treating hypothyroidism. That evidence is about replacing a hormone someone lacks.
The label position is a boxed warning, not an absence of evidence: thyroid hormone must NOT be used for obesity or weight loss. At replacement-range amounts it is ineffective in people with normal thyroid function, and at larger amounts it can cause serious or life-threatening toxicity. There is no safe, clinically acceptable weight-loss use — which is a different and stronger statement than saying it cannot reduce weight.
Studies of supraphysiologic thyroid hormone show accelerated loss of lean mass alongside fat, meaning the composition of the weight lost is unfavourable.
Excess thyroid hormone is associated with atrial fibrillation and with reduced bone density over time — both well documented in the endocrine literature on over-replacement.
Over-the-counter "thyroid support" products have repeatedly been found on analysis to contain actual thyroid hormone, undeclared, at clinically active amounts.

Half-life and how long it lasts

T4 has a long half-life of about a week, so changes take weeks to show and weeks to wash out. T3 has a half-life of roughly a day, acts fast and swings hard, which is what makes it more dangerous in misuse. Suppression of your own thyroid axis develops over weeks and takes a comparable or longer period to recover.

Risks

Excess exogenous thyroid hormone can cause iatrogenic thyrotoxicosis: muscle catabolism (a meaningful share of the weight lost is tissue you wanted to keep), bone-density loss, arrhythmia and cardiac strain, heat intolerance, anxiety. Exogenous hormone suppresses your own axis; transient suppression and hypothyroid-pattern symptoms can follow stopping, with recovery time varying. Unapproved thyromimetics add unknown-compound risk on top.

Who should never touch it

Anyone with normal thyroid function — which is the entire misuse case
Anyone with heart disease, arrhythmia or a history of atrial fibrillation
Anyone with osteoporosis or reduced bone density
Anyone stacking stimulants or beta-agonists
Anyone with uncorrected adrenal insufficiency — an absolute labelled contraindication, because starting thyroid hormone speeds clearance of glucocorticoids and can precipitate an adrenal crisis
Anyone over 60, where cardiac and bone risks rise sharply

Interactions worth knowing

Beta-agonists such as clenbuterol — a common and particularly dangerous stack, since both raise cardiac demand.
Any stimulant, for the same reason.
Anticoagulants, whose effect thyroid hormone potentiates.
Calcium, iron and some antacids, which block absorption and must be separated by hours.
Diabetes medication, since thyroid status alters insulin requirements.

Stacking interactions

Documented interactions with other things people research. A compound not listed here does not mean the combination is safe — only that no specific interaction is on file.

CautionClenbuterol