An unusual case of inappropriate secretion of thyrotropin: neoplastic or nonneoplastic?

OBJECTIVE: To report an unusual case that illustrates the difficulties in distinguishing neoplastic (nIST) from nonneoplastic inappropriate secretion of thyrotropin (nnIST). METHODS: We describe clinical, biochemical, genetic analysis, and magnetic resonance imaging (MRI) results in a patient with hyperthyroidism due to IST, and we review the related literature. RESULTS: The patient demonstrated overt clinical and biochemical hyperthyroidism with inappropriately increased thyrotropin levels, which failed to respond to thyrotropin-releasing hormone (TRH) stimulation. Sex hormone-binding globulin (SHBG) levels were in the hyperthyroid range. Alpha subunit levels were normal, as was the alpha subunit/thyrotropin molar ratio. MRI of the pituitary was negative for tumor during a 2-year period, and octreotide scan was also negative for sellar uptake. Basal oxygen consumption was abnormally increased. Genetic analysis failed to reveal mutations of the thyroid receptor b gene. The patient responded well to radioiodine ablation of his thyroid. CONCLUSION: This patient had clinical symptoms of hyperthyroidism associated with some features characteristic of nIST (increased level of SHBG, lack of thyrotropin response to TRH stimulation, absence of thyroid receptor b mutations) and others typical of nnIST (normal alpha subunit and its molar ratio to thyrotropin, absence of tumor on sellar imaging). Close follow-up with periodic MRI of the sella is important because of the possible existence of a small pituitary tumor, which may become apparent at a later date. Therapy to control symptoms is important. Hormone replacement, if needed, should be adjusted to maintain clinical euthyroidism, guided by free thyroxine levels.