RESEARCH MONOGRAPH · KDC-MN-288
Liothyronine (T3)
Liothyronine (Cytomel) is pure synthetic T3, the active form of thyroid hormone. The standard hypothyroidism drug, levothyroxine, is T4, which the body slowly converts to T3 in target tissues. T3 acts directly on thyroid receptors and produces faster onset and stronger effects, but with a much shorter half-life (about 2.5 days versus 7 days for T4), giving more variable plasma levels. It is used in hypothyroidism (sometimes added to T4 in patients who do not feel well on T4 alone), as augmentation in treatment-resistant depression (a long-standing psychiatry tactic), and off-label by bodybuilders for fat loss, where it can suppress endogenous thyroid output and cause cardiac and bone problems. Not stocked by Kodiac. This monograph is provided for research and educational reference.
Intrigue 0–100 blends mechanism novelty, evidence strength, and translational potential. Kodiac editorial, not peer-reviewed.
Active thyroid hormone (T3)
The active thyroid hormone triiodothyronine; faster-acting and more potent than levothyroxine, used in hypothyroidism, depression augmentation, and metabolic research.
Abstract
Liothyronine (3,3',5-triiodo-L-thyronine, T3; CAS 6893-02-3; molecular formula C15H12I3NO4; molecular weight 650.97) is the active thyroid hormone, marketed as Cytomel for hypothyroidism. The compound binds thyroid hormone receptors (TR-alpha, TR-beta) with approximately 4-fold higher affinity than thyroxine (T4) and produces direct genomic effects on metabolic gene transcription without the conversion delay required for T4. Plasma half-life is 2.5 days, much shorter than T4's 7-day half-life, producing more variable plasma levels but faster onset and offset. Approved for hypothyroidism (alternative or adjunct to levothyroxine), myxedema coma (intravenous), and as a thyroid suppression test. Off-label use in major depressive disorder augmentation (the STAR*D trial-supported strategy of T3 25 to 50 mcg added to a partially-responding antidepressant), fat loss, and bodybuilding cutting cycles. The cardiac risk is the principal limitation: T3 directly increases myocardial oxygen demand, producing tachycardia, atrial fibrillation, and angina at supratherapeutic doses. Used as the canonical T3 reference compound.
Mechanism of action
Active thyroid hormone; binds TR-alpha and TR-beta with ~4-fold higher affinity than T4. Direct genomic effects without T4-to-T3 conversion delay.
Reported research dose ranges
Clinical 5 to 75 mcg in the published literature. Depression augmentation 25 to 50 mcg. Off-label fat loss 25 to 75 mcg.
References
- Wiersinga WM, et al. Combined therapy with levothyroxine plus liothyronine in hypothyroidism. Nat Rev Endocrinol 2014.
- Aronson R, et al. Triiodothyronine augmentation in the treatment of refractory depression: a meta-analysis. Arch Gen Psychiatry 1996.
- Joffe RT, et al. Triiodothyronine in depression: a meta-analysis. Psychiatry Res 2010.
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The full reference document is provided strictly for research use only. It reports research dose ranges from the published literature, not instructions for use in humans or animals.
FOR RESEARCH USE ONLY. Not for medical, diagnostic, or therapeutic purposes. Not for human consumption. All information is provided for research and educational purposes only.