Follow-up studies of a woman with hyperthyroidism due to excessive TSH secretion, previously reported (1), are described. For most of the ensuing years, she has been treated with methimazole. On several occasions, methimazole was discontinued; each time elevations of serum TSH, T4, and T3 concentrations, similar in magnitude to those originally present, occurred. Initially, her serum TSH concentrations fell substantially after dexamethasone administration and did not change after TRH. The same responses have occurred on several occasions during the follow-up period. Serum glycoprotein alpha-subunit and TSH-beta concentrations were normal and did not change after TRH administration, but serum alpha-subunit declined during dexamethasone and increased slightly after gonadotropin-releasing hormone. No increase in serum TSH occurred after TRH administration when TSH secretion was decreased by dexamethasone administration. No evidence of pituitary enlargement has developed during the 7-yr follow-up period. The cause of this patient's excessive TSH secretion remains unknown, but her TSH secretory dynamics are most consistent with those found in patients who had TSH-secreting pituitary adenomas reported in recent years.