Liothyronine
Liothyronine, a hormone. Active thyroid hormone — ~4× more potent than T4 but ~10% the plasma half-life. Binds nuclear TRα/TRβ to regulate… Oral half-life 24 h.
Active thyroid hormone — ~4× more potent than T4 but ~10% the plasma half-life. Binds nuclear TRα/TRβ to regulate metabolic rate, heart rate, thermogenesis. T4 → T3 conversion occurs in peripheral tissues via deiodinases (selenium-dependent). Direct T3 replacement bypasses deiodinase variability.
PK: Nicoloff 1972 (PMID:4110897), simultaneous T3/T4 tracer kinetics in 31 subjects, verbatim "The normal mean T(3) fractional turnover rate (kT(3)) was 0.68 (half-life = 1.0 days)" and "The mean T(3) distribution space in normal subjects was 38.4 liters" — the volume stored until 2026-09-06 was 28 L, which appears in no paper. The previous citation (PMID:17259789) is a relative-bioequivalence study of oral formulations that reports only Cmax, AUC and Tmax, and has NO IV arm despite having been cited for the IV route. ka and F dropped as unsourced; the observable is Tmax 2.5 h after 50 mcg oral (PMID:24977379). CAVEATS: distribution equilibration takes ~22 h against a 24 h half-life, so a one-compartment curve under-predicts the peak; and endogenous production is ~27.6 mcg/day, the same order as a 25-75 mcg dose, so in a euthyroid user this is a perturbation of an occupied pool under TSH feedback, not a curve rising from zero.
Pharmacokinetics
- Oral — t½ 24 h · Vd 38.4 L
- IV — t½ 24 h · F 100% · Vd 38.4 L
Related compounds
- Selenium — synergistic
- Iodine — synergistic
- Levothyroxine — synergistic
Sources: PMID:17259789, PMID:4110897, PMID:24977379