Clinical and Hormonal Characteristics of Patients with Syndrome of Inappropriate Secretion of Thyrotropin

Purpose: Normal or elevated thyrotropin (TSH) levels in the presence of elevated thyroxine is defined as syndrome of inappropriate secretion of TSH. The two main clinical conditions that can lead to this syndrome are TSH-secreting adenoma (TSHoma) and resistance to thyroid hormone. Establishing the correct diagnosis is crucial in order to decide on the most appropriate treatment option. Herein, we present the data of seven patients who were hospitalized for the differential diagnosis of the two clinical entities.\ud Material and Method: Our database was reviewed for patients diagnosed with syndrome of inappropriate secretion of TSH in our hospital between 2010 and 2014. After exclusion of the other rare causes, seven patients who were hospitalized for the differential diagnosis of TSHoma and resistance to thyroid hormone were included.\ud Results: The final diagnosis was resistance to thyroid hormone in four patients, TSHoma in two and equivocal in one. Two patients diagnosed with TSHoma were operated and had positive staining with TSH. Both of the TSHoma cases had macroadenoma on pituitary magnetic resonance imaging and visual field defect, while two of four patients with resistance to thyroid hormone had microadenoma. Alpha-subunit/TSH molar ratio was above 1 in all patients diagnosed with TSHoma while it exceeded 1 in two patients with the final diagnosis of resistance to thyroid hormone. Thyrotrophin-releasing hormone stimulation test revealed a blunted response in all patients with TSHoma and a positive response in all with resistance to thyroid hormone.\ud Discussion: Differential diagnosis of resistance to thyroid hormone and TSHoma can be a clinical challenge and requires complex hormone testing and imaging methods. Since incidental pituitary tumors are not rare, presence of an adenoma should not rule out the diagnosis of resistance to thyroid hormone