The presence at presentation of hyperprolactinaemia and abnormalities on high resolution computed tomographic (CT) scanning of the pituitary gland have been documented in 19 patients with primary hypothyroidism. The changes on thyroid hormone replacement were followed for up to 28 months after presentation. At presentation serum PRL was elevated in 12 subjects (454 to 2612 mU/l); after stabilization of thyroid replacement PRL remained raised in 9 patients (525 to 1888 mU/l) despite 10-18 months of treatment. Pretreatment CT scans showed enlarged pituitaries in 11 patients with suprasellar extension in five. In 11 patients there was initially an area of low attenuation suggesting a microadenoma and on rescanning this appearance remained in five patients. Hyperprolactinaemia in hypothyroid patients does not always return to normal with thyroid hormone replacement.