Liothyronine (T3)
Liothyronine is synthetic triiodothyronine (T3), the biologically active thyroid hormone, prescribed for hypothyroidism and myxoedema coma. It is used off-label for fat loss because it directly raises metabolic rate, but supraphysiological use causes muscle loss, cardiac strain and bone loss, and abruptly stopping it can cause a rebound suppression of the thyroid axis.
01 Overview
Liothyronine is the sodium salt of T3, the active thyroid hormone that regulates basal metabolic rate across nearly all tissues. Clinically it is used less often than levothyroxine (T4) because of its short half-life and sharper peaks, but it acts faster and more potently. Standard replacement doses are small and titrated to thyroid function tests.
Off-label, T3 is used to accelerate fat loss by pushing metabolic rate above normal. Because thyroid hormone is catabolic to muscle as well as fat, supraphysiological doses cause loss of lean mass alongside fat, and impose cardiac strain (tachycardia, arrhythmia) and accelerated bone turnover. Exogenous T3 suppresses endogenous thyroid output, so use should generally be tapered rather than stopped abruptly.
02 Mechanism
Directly activates nuclear thyroid hormone receptors as the active hormone T3, upregulating metabolic gene expression and increasing basal metabolic rate, oxygen consumption and substrate turnover across tissues.
03 Dosing
| Tier | Dose | Route | Notes |
|---|---|---|---|
| Replacement | 25–50 mcg/day | Oral | Typical hypothyroid replacement, titrated to labs. |
| Off-label fat loss | 50–75 mcg/day | Oral | Supraphysiological off-label range; increasing muscle-loss and cardiac risk with dose. |
| High off-label | 100 mcg/day | Oral | High doses markedly raise cardiac and catabolic risk. |
04 Effects
| Effect | Magnitude | Evidence | |
|---|---|---|---|
| Increased metabolic rateDirect dose-dependent increase in basal metabolic rate and fat oxidation. | dose-dependent | Clinical | |
| Faster fat lossAccelerated weight loss when combined with a caloric deficit, though not fat-selective. | accelerated in deficit | Observational |
05 Side effects
| Effect | Severity | Frequency | Evidence | Countermeasures |
|---|---|---|---|---|
| Muscle lossThyroid hormone is catabolic to lean tissue; supraphysiological T3 accelerates loss of muscle as well as fat. | Moderate | With supraphysiological doses | Clinical | |
| Cardiac strain and arrhythmiaTachycardia, palpitations and increased risk of atrial fibrillation with excess T3, particularly in older users. | Severe | Dose-dependent | Clinical | |
| Bone density lossProlonged thyroid excess accelerates bone turnover and can reduce bone mineral density. | Moderate | With prolonged excess | Clinical | |
| Rebound thyroid suppressionExogenous T3 suppresses endogenous thyroid output; abrupt withdrawal causes transient hypothyroid symptoms until the axis recovers. | Moderate | On abrupt cessation | Clinical |