Pituitary hyperplasia mimicking thyrotropin-producing pituitary adenoma in the patient with resistance to thyroid hormone: a case report.

The syndrome of inappropriate thyroid-stimulating hormone (TSH) secretion is characterized by high circulating thyroid hormone concentrations in the presence of non-suppressed TSH. After exclusion of the laboratory interference, TSH-secreting pituitary adenoma (TSHoma) or resistance to thyroid hormone-beta (RTH-β) should be suspected. The presence of a pituitary adenoma on the hypothalamic-pituitary imaging supports the diagnosis of TSHoma. However, the incidental findings of non-functioning pituitary adenomas may appear in patients with RTH-β. Abnormal MRI finding in the RTH-β patient also includes pituitary enlargement from thyrotroph hyperplasia. We herein reported a patient with inappropriate TSH secretion who has pituitary hyperplasia mimicking TSHoma. This case illustrates the diagnostic tests to distinguish an RTH-β from TSHoma.