T3 (liothyronine)

What this is

Often sought by patients who feel unwell on T4 monotherapy despite normal TSH. AACE/ATA 2014 guidelines acknowledge T3 combination therapy as reasonable. Short half-life (~1 day) typically requires divided dosing. Off-label fat-loss use (supraphysiologic 25-100 mcg/day) is dangerous and not a sound strategy in euthyroid individuals: it suppresses endogenous thyroid function, drives muscle catabolism alongside fat loss, and discontinuation often causes rebound hypothyroidism — the cardiac (arrhythmia) and bone-loss consequences are the binding constraint.

Mechanism

Biologically active thyroid hormone; binds thyroid hormone receptors (TRα, TRβ); upregulates metabolic rate, lipolysis, protein turnover; shorter half-life (~1 day) than T4

Dose & route

Clinical: 5-25 mcg/day, often divided, in T4/T3 combo therapy; supraphysiologic 25-100 mcg/day seen in off-label fat-loss / bodybuilding cutting use (not advised)

Citations

• https://pubmed.ncbi.nlm.nih.gov/25266247/
• https://pubmed.ncbi.nlm.nih.gov/17283286/
• https://pmc.ncbi.nlm.nih.gov/articles/PMC6031714/
• https://pubmed.ncbi.nlm.nih.gov/40917329/
• https://pubmed.ncbi.nlm.nih.gov/40851876/
• https://pubmed.ncbi.nlm.nih.gov/16670164/
• https://pubmed.ncbi.nlm.nih.gov/23565056/
• https://www.thyroid.org/hyperthyroidism/
• https://pubmed.ncbi.nlm.nih.gov/40700575/

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