A 30 year old woman with primary hypothyroidism due to an ectopic (sublingual) thyroid, with a pituitary tumour invasive to the sphenoidal sinus presented with hyperprolactinaemia, amenorrhoea and galactorrhoea not corrected by long-term thyroid replacement. High basal TSH and TRH hyperresponsiveness were suppressed by thyroid therapy. After transsphenoidal removal of the tumour the Prl returned to normal. Light microscopy including differential staining and immunocytochemistry demonstrated that the tumour was composed of TSH cells. Ultrastructurally the adenoma cell contained small granules (90-150 nm) as described in TSH cells. This case provides evidence that a TSH cell adenoma probably arising from protracted overstimulation secondary to thyroid deficiency can induce hyperprolactinaemia. This did not appear to be produced directly by the adenoma nor was there any evidence of compression of the pituitary stalk.