Five female patients with primary hypothyroidism and radiological evidence of a pituitary enlargement were studied before and after a mean of 30 months (range 12–83 months) treatment with thyroxine (T<sub>4</sub>). Before treatment, serum thyroid-stimulating hormone (TSH) levels were elevated in every patient (mean 392 mU/l, range 240–475) and prolactin levels in 4 (mean 79 µg/l, range 48–143 µg/l). CT scanning confirmed the presence of pituitary enlargement in the 4 patients studied, which was suprasellar in 3. The remaining patient had an enlarged fossa on a lateral skull radiograph. During treatment with T<sub>4</sub>, TSH and prolactin levels were normal in all. Complete disappearance of the enlargement was seen on follow-up scans in all patients and 1 developed an empty sella. The induction of a pituitary enlargement by primary hypothyroidism results from reversible hyperplasia of both the TSH and prolactin-secreting cells in most instances. Occasionally, however, hyperplasia of the thyrotrophs can occur in isolation and an empty sella can occur after successful treatment with T<sub>4</sub>. Thyroid function tests should be obtained in all hyperprolactinemic patients.