A Case of Thyrotropin-Secreting Pituitary Microadenoma with Normal Thyrotropin α-Subunit Level

We present a 32-year-old male with a thyrotropin (TSH)-secreting pituitary microadenoma with normal α-subunit (SU) and/or α-SU/TSH molar ratio. An interesting feature of this patient is that the size of the pituitary tumor remained unchanged during a 6-year follow-up without treatment. The tumor was clearly visualized with somatostatin receptor imaging, indicating that it was somatostatin receptor-positive. Subcutaneous injection of 100 μg octreotide acetate three times daily resulted in significant reduction of TSH and free thyroid hormones 6 weeks after initiation of treatment. However, tumor size was not changed 3 months after initiation of octreotide therapy and thyroid hormones, but not TSH level, eventually increased in spite of increasing the octreotide dosage up to 600 μg/day. This led to discontinuation of treatment. The patient responded only temporarily to octreotide in spite of somatostatin receptors. This case further demonstrates that a normal α-SU and/orα-SU/TSH molar ratio cannot exclude the possibility of a TSH-secreting pituitary adenoma.