Hypersecretion of TSH by a pituitary adenoma is thought to be a rare form of hyperthyroidism. We describe three such patients, each of whom presented with clinical hyperthyroidism and a diffuse goitre, without eye signs or dermatopathy. Two were receiving long-term phenothiazine, one of these was also acromegalic. Plasma thyroxine, free T4 and tri-iodothyronine were repeatedly raised in each; plasma TSH was grossly elevated in one and inappropriately normal in the other two. Plasma TSH did not rise in response to thyrotrophin-releasing hormone or metoclopramide and was not suppressed by L-dopa in any patient. Anti-thyrotrophin receptor antibodies were undetectable. Skull ra0diographs showed erosion and expansion of the pituitary fossa and CT scans confirmed a pituitary mass in each patient. A pituitary adenoma was removed by trans-sphenoidal surgery in two patients and a TSH-secreting adenoma was confirmed by immunocytochemical staining and electron microscopy. Both patients were clinically euthyroid post-operatively but still had evidence of TSH excess. Pituitary surgery was technically unsuccessful in the third patient. Although two patients had hyperthyroidism of long duration, all three were diagnosed within one year of the introduction of a sensitive TSH assay to our laboratory. A TSH-secreting pituitary adenoma may be a more common cause of hyperthyroidism than has been believed. © 1990, Oxford University Press.