The authors detected in a 30-year-old patient a very rare type of hyperthyroidism caused by a thyrotropin secreting pituitary adenoma. Scintigraphic examination of the pituitary by means of 111In radiolabelled octreotide revealed an increased accumulation of the radiopharmaceutical preparation in the tumour, which confirmed the high density of somatostatin receptors. After onset of octreotide treatment (Sandostatin, Sandoz, Switzerland) 3 x 100 ug/day by the s.c. route a brisk decline and normalization of thyrotropin already after the first dose was recorded. The thyroxine concentration declined slowly to the upper range of normal values. After 5 months treatment despite the positive response to receptor scintigraphy diminution of the adenoma was not recorded. Again an increase of thyrotropin above the upper limit of the reference range and a marked rise of thyroxinaemia were observed. Six months after radical selective trans-sphenoidal adenomectomy normal pituitary function was confirmed.