SUMMARYA 20–year‐old female presented with thyrotoxicosis associated with amenorrhoea and galactorrhoea after oral contraceptive withdrawal. Serum TSH was persistently elevated (mean: 28 ± 31 μU/ml during 24–h sampling and did not vary significantly after TRH, bromocriptine or somatostatin. Prolactin levels remained constantly at the upper limit of normal (mean: 206 ± 21 ng/ml, with a minimal increase after TRH, a slight decrease after somatostatin (54%) and a marked decrease after bromocriptine (885%). Surgical exploration revealed an unusually firm tumour adherant to the wall of the sella turcica; electron microscopic study showed that it was composed almost exclusively of thyrotrophs. After a transient period of euthyroidism post‐operatively, T3 toxicosis occurred with an increased TSH level (155 μU/ml) and a rise in TSH and alpha subunit in response to TRH. An increase in T4 followed while prolactin r