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 pit uitary microadenoma is described. She had a high serum free T-4 concen tration (31 pmol/L) with an inappropriately nonsuppressible serum TSH concentration (0.93 mU/L). The alpha/TSH molar ratio was 2.3 and magne tic resonance imaging with gadolinium enhancement identified an area o f low signal intensity in the left lateral pituitary gland. However, T SH secretion was not completely autonomous. There was a significant re sponse to exogenous TRH stimulation and suppression by T-3 administrat ion. Therefore, it was difficult to rule out a nonfunctioning pituitar y adenoma with concomitant pituitary selective thyroid hormone resista nce syndrome. A 2-mm microadenoma was excised via transsphenoidal surg ery. The tumor cells were immunoreactive to antisera to cy-subunit and minimally immunoreactive to antisera to TSH beta. The patient's thyro id function normalized after surgery without medication. Because the a denoma could become large and intractable if the patient was treated i nadequately, early diagnosis and treatment are important in patients w ith TSH secreting adenomas.