Levothyroxine

Category: hormone

Aliases: L-thyroxine, T4, Synthroid, Levoxyl

Pharmacological Mechanism

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 1.6mcg (Range: 1.6 - 1.6 mcg)
• IV: Typical 1.6mcg (Range: 1.6 - 1.6 mcg)

Target Organ Systems

• endocrine

Interactions

• Calcium
• Iron
• Liothyronine (T3)
• Cholestyramine