USMLE (United States Medical Licensing Examination) Step 1 Practice Exam

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What phenomenon occurs in patients with iodine deficiency and partially autonomous thyroid tissue when made iodine replete, leading to thyrotoxicosis?

Jod-Basedow phenomenon
Nelson Synd
Pseudohypoparathyroidism type 1A (Albright hereditary osteodystrophy)
Pheochromocytoma

The correct answer is: Jod-Basedow phenomenon

The phenomenon that occurs in patients with iodine deficiency and partially autonomous thyroid tissue when they are made iodine replete, leading to thyrotoxicosis, is known as the Jod-Basedow phenomenon. When iodine becomes available in sufficient quantities, the hyperactive thyroid tissue, which has developed due to insufficient iodine intake and has gained the ability to produce thyroid hormones autonomously, can suddenly release excessive amounts of thyroid hormones into the bloodstream. This results in thyrotoxicosis, characterized by increased levels of circulating thyroid hormones leading to symptoms such as weight loss, heat intolerance, and palpitations. The other options do not relate to this specific scenario. Nelson syndrome involves pituitary adenomas that occur due to the loss of feedback inhibition after adrenalectomy, which is unrelated to thyroid dynamics. Pseudohypoparathyroidism type 1A is a genetic disorder related to a defect in the G protein signaling pathway, affecting parathyroid hormone signaling rather than thyroid hormone production. Pheochromocytoma is a tumor of the adrenal medulla producing catecholamines, leading to symptoms associated with increased adrenergic activity, and is not linked to thyroid hormone levels.