A pituitary tumor in a patient with thyroid hormone resistance: A diagnostic dilemma

Resistance to thyroid hormone (RTH) is due to mutations in the β-isoform of the thyroid hormone receptor (TR-β). RTH patients display inappropriate secretion of thyrotropin-releasing hormone (TRH) from the hypothalamus and thyrotropin (TSH) from the anterior pituitary, despite elevated levels of thyroid hormone thyroxine (T₄) and triiodothyronine (T₃). Thyrotropin-secreting tumors are presumed to represent clonal expansion of abnormal cells. Because the diagnosis of TSH-secreting tumors tends to be delayed and curative surgical resection remains under 50%, early diagnosis is paramount. Current diagnostic strategies suggest that RTH patients are distinguishable from patients with TSH-secreting pituitary tumors by the use of standard laboratory tests and imaging. Here, we present a woman in whom the standard evaluation for inappropriate TSH secretion was insufficient to distinguish these entities. The patient had a low-normal TRH stimulation test and an unmeasurable α-glycoprotein subunit level; however, a pituitary magnetic resonance imaging (MRI) revealed an adenoma. More testing using a T₃ suppression test supported a RTH diagnosis and a R438H mutation was found in the TR-β gene. To our knowledge, this represents the first report of an apparently incidental pituitary adenoma in the setting of documented resistance to thyroid hormone. As such, it raises the question of whether RTH predisposes to pituitary hyperplasia and adenoma development.