ABSTRACT

Thyrotropin (TSH)-secreting pituitary adenomas are rare tumors that indicate an uncommon cause of hyperthyroidism. The diagnosis of a TSH-secreting pituitary adenoma can present a challenge, and may be difficult to distinguish from thyroid hormone resistance (RTH). TSH-secreting adenomas usually have a disproportionate hypersecretion of the alpha subunit of glycoprotein hormones, leading to an elevated molar ratio of serum alpha subunit to TSH. Resistance to thyroid hormone is a rare inherited disorder characterized by the reduced responsiveness of target tissues to thyroid hormone and can result in a wide range of clinical features from hypothyroidism to hyperthyroidism. A broad range of clinical symptoms have been reported with RTH but most commonly patients present without severe clinical symptoms despite the abnormal thyroid function tests. Interestingly, clinical symptoms vary widely from patient to patient even within families who harbor the same mutation. Differentiated thyroid cancer metastases can rarely produce excess thyroid hormone and result in hyperthyroidism.