What is the approximate half-life of levothyroxine?

• A. About one day
• B. About several hours
• C. About one week
• D. About one month

Answer: (C)

Levothyroxine has a long half-life, meaning it stays in the body for a relatively extended period before its levels drop by half. For a healthy adult, the approximate half-life is about seven days. This long half-life fits with how dosing is managed in practice: changes take several weeks to reflect fully because it takes roughly 4–6 weeks to reach a new steady state after adjusting the dose. A much shorter half-life (about a day or several hours) would imply rapid clearance, which isn’t characteristic of levothyroxine, while a much longer half-life (about a month) would delay adjustments far more than is typical. So, about one week is the best approximation for its half-life.