Name: Levothyroxine sodium (Synthroid, Levothroid, Levoxine)
Class: Thyroid Hormone
Mechanism: Sodium salt of T4. T3/T4 synergize w/GH effects, increase BMR, potentiate catecholamine effects on heart, promote lipolysis, and decrease serum cholesterol.
Absorption: Incomplete oral absorption—30-40% recovered in stool.
Dist.: Poor placental transfer ® okay for pregnant E. Little in milk (use cautiously).
Metabolism.: Hepatic conjug. w/glucuronic & sulfuric acids. Peripheral deiodination to T3.
Excretion, t½: Bile; some lost in stool due to enterohepatic circ. 6-7 d.
Toxicity/S.E.s: Salicylates and dicumarol compete for albumin binding sites ® marked increase of free levels.
Utility: Treat hypothyroidism and goiter (not due to iodine deficiency or hyperthyroidism).
Special Features: 200-300 mg/d. 1/4 the potency of liothyronine sodium, but same efficacy.