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.
Half-Life (t½)
PO: 24h, IV: 24h
Dosing Guidelines
PO: Typical 41.7 mcg (Range: 25–75 mcg)
IV: Typical 41.7 mcg (Range: 25–75 mcg)
Target Organ Systems
endocrine
Interactions
Selenium (synergistic): Deiodinase cofactor for T4 → T3 conversion.
Iodine (synergistic): Structural component of all thyroid hormones.
Levothyroxine (synergistic): Combination T4/T3 therapy in persistent symptoms despite normal TSH; evidence mixed.