We present a 45-yr-old male with clinical signs and symptoms of mild hyperthyroidism, high serum levels of T₃, T₄, and FT₄ as well as serum TSH concentration. The elevated α-subunit level and α-subunit/TSH molar ratio were also observed. These findings indicated the presence of hyperthyroidism due to inappropriate secretion of TSH, whose neoplastic origin was documented by nuclear magnetic resonance scan showing a 0.6 cm pituitary adenoma. Selective pituitary adenomectomy was completely successful; α-subunit, TSH, T₃, T₄, and FT₄ normalized, and euthyroidism was restored. Light microscopic immunohistochemistry showed that the adenoma was composed of TSH-secreting cells