Pharmacology · single question
What is the mechanism of action of levothyroxine?
Show answer and explanation
Correct answer: D. T4 replacement (exogenous thyroid hormone)
- A. Levothyroxine itself IS T4 and undergoes peripheral conversion to T3 — it does not inhibit this conversion. Propylthiouracil (PTU) inhibits T4 to T3 conversion (in hyperthyroid treatment).
- B. Levothyroxine actually SUPPRESSES TSH (via negative feedback on the pituitary) — it does not stimulate TSH. TSH stimulates the thyroid gland to produce thyroid hormone.
- C. Iodine uptake by the thyroid is stimulated by TSH — not by levothyroxine. Levothyroxine bypasses the thyroid gland entirely.
- D. Levothyroxine (Synthroid) is SYNTHETIC T4 (thyroxine) — it directly REPLACES the body's deficient thyroid hormone in hypothyroid patients. It is converted peripherally to T3 (the active form). It acts on nuclear thyroid hormone receptors to regulate metabolism, growth, and protein synthesis.