Synthetic L-isomer of thyroxine (T4). Peripherally deiodinated by DIO1/2 to active T3 — provides physiologic hormone with controlled release. Narrow therapeutic index; TSH-guided titration every 6–8 weeks. Absorption disrupted by many things — take fasting separate from calcium/iron/PPIs/antacids.
Half-Life (t½)
PO: 168h, IV: 168h
Dosing Guidelines
PO: Typical 1.6 mcg (Range: 1.6–1.6 mcg)
IV: Typical 1.6 mcg (Range: 1.6–1.6 mcg)
Target Organ Systems
endocrine
Interactions
Calcium (caution): Reduces T4 absorption ~30% — separate by ≥4 h.
Iron (caution): Same chelation mechanism — separate by ≥4 h.
Liothyronine (T3) (synergistic): Combination T4/T3 therapy in persistent symptoms despite normal TSH; evidence mixed.