Why must iodine therapy (Lugol iodine or sodium iodide) be withheld until at least one hour after administration of a thionamide in the treatment of thyroid storm?
- A Iodine inactivates thionamides if given concurrently
- B Iodine provides substrate for new hormone synthesis unless organification is already blocked ✓
- C Iodine precipitates cardiac arrhythmias if given before beta blockade
- D Thionamides increase renal clearance of iodine when given together
Correct answer: B. Iodine provides substrate for new hormone synthesis unless organification is already blocked
Explanation
Iodine acutely inhibits hormone release (Wolff-Chaikoff effect) but also supplies substrate for new thyroxine synthesis. If organification by the thyroid peroxidase enzyme has not already been blocked by a thionamide, the iodine load can fuel further hormone production and worsen the storm. Therefore the thionamide is always given first, with iodine following at least an hour later. Beta blockers are given independently and do not determine iodine timing.
Reference: Katzung Basic and Clinical Pharmacology, 15th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.