Vignette: A 32-year-old woman presents to her primary care physician complaining of bothersome hand tremors for the past month. She also reports unintentional weight loss, heat intolerance, and increased frequency of bowel movements. On physical examination, her heart rate is 102/min and blood pressure is 130/85 mm Hg. There is a mild tremor in her outstretched hands. The thyroid gland is diffusely enlarged without any nodules. The physician suspects hyperthyroidism and prescribes a medication that inhibits the organification of iodide and coupling of thyroid hormone synthesis. Which of the following is the most likely medication?
D) Radioactive iodine
E) Iodine solution
The clinical features are suggestive of hyperthyroidism, which is most commonly caused by Graves' disease. Methimazole and propylthiouracil (PTU) are the first-line treatment for hyperthyroidism. They inhibit the thyroid peroxidase enzyme, thereby blocking iodide organification and coupling of monoiodotyrosine (MIT) and diiodotyrosine (DIT) residues, which are necessary steps in thyroid hormone synthesis. Although PTU also inhibits the peripheral conversion of T4 to T3, methimazole is generally preferred because of its longer half-life and lower incidence of side effects. The other options listed, levothyroxine, radioactive iodine, and iodine solution, are not used primarily to inhibit thyroid hormone synthesis. Levothyroxine is a synthetic form of T4 used in hypothyroidism, radioactive iodine is used to ablate the thyroid gland in hyperthyroid patients who are not surgical candidates, and iodine solution can be used preoperatively to decrease thyroid gland vascularity.