A patient with primary hypothyroidism is started on oral levothyroxine as a single daily dose rather than divided doses. Which pharmacodynamic property of thyroid hormones justifies once-daily dosing?
- A Complete bioavailability of levothyroxine exceeding 95 percent
- B Half-life of approximately 7 days for T4 versus about 1 day for T3 ✓
- C Irreversible binding of T4 to its nuclear receptor
- D Rapid equilibration of serum T4 with intracellular T3 within hours
Correct answer: B. Half-life of approximately 7 days for T4 versus about 1 day for T3
Explanation
Levothyroxine is a prohormone converted peripherally to T3, and its long plasma half-life of about 7 days keeps steady-state levels stable with one daily dose, whereas T3 with a half-life near 1 day would require divided dosing. Levothyroxine bioavailability is only about 70 to 80 percent and varies with food and drugs, killing option A. Receptor binding is reversible, eliminating option C.
Reference: Katzung's Basic and Clinical Pharmacology, 16th ed.
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