Thyrotropin-secreting pituitary microadenoma

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

Resistance to thyroid hormone (RTH) is due to mutations in the beta-isoform of the thyroid hormone receptor (TR-beta). 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 alpha-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-beta 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.

Difficulty in differentiating thyrotropin secreting pituitary microadenoma from pituitary-selective thyroid hormone resistance accompanied by pituitary incidentaloma

A 33-year-old woman with inappropriate secretion of TSH and a 2-mm pituitary microadenoma is described. She had a high serum free T4 concentration (31 pmol/L) with an inappropriately nonsuppressible serum TSH concentration (0.93 mU/L). The alpha/TSH molar ratio was 2.3 and magnetic resonance imaging with gadolinium enhancement identified an area of low signal intensity in the left lateral pituitary gland. However, TSH secretion was not completely autonomous. There was a significant response to exogenous TRH stimulation and suppression by T3 administration. Therefore, it was difficult to rule out a nonfunctioning pituitary adenoma with concomitant pituitary selective thyroid hormone resistance syndrome. A 2-mm microadenoma was excised via transsphenoidal surgery. The tumor cells were immunoreactive to antisera to alpha-subunit and minimally immunoreactive to antisera to TSHbeta. The patient's thyroid function normalized after surgery without medication. Because the adenoma could become large and intractable if the patient was treated inadequately, early diagnosis and treatment are important in patients with TSH secreting adenomas.