Co-occurrence of thyroid binding globulin excess and a pituitary macroadenoma containing thyroid stimulating hormone-producing cells.

A patient is described in whom thyroid binding globulin (TBG) excess was found in association with a pituitary macroadenoma containing thyroid stimulating hormone (TSH)-producing cells, and the potential for diagnostic confusion arising from this unusual combination of endocrine disorders is discussed. The patient presented with visual field disturbance, and magnetic resonance imaging led to the diagnosis of a pituitary tumour. Raised levels of total thyroxine and triiodothyronine along with normal TSH levels suggested the possibility of a TSH-secreting pituitary adenoma. Immunostaining of the tumour removed at transsphenoidal surgery confirmed the presence of TSH-producing cells. When raised total thyroxine and triiodothyronine levels persisted postoperatively, the diagnosis of TBG excess was confirmed. Use of free, rather than total, thyroid hormone assays would almost certainly have prevented the diagnostic confusion concerning possible TSH-secreting pituitary adenoma, but may have resulted in the TBG excess being overlooked entirely.