An 11 year old boy was found to have hyperthyroidism and elevated serum TSH concentrations. Hyperthyroidism was first diagnosed at 4 years of age. After antithyroid drug therapy, a subtotal thyroidectomy was done when he was 7 years old. Goiter and hyperthyroidism gradually recurred, and an elevated serum TSH concentration (90 microU/ml) was found when first measured at age 12 years. There was no evidence of a pituitary tumor. Thyrotropin-releasing hormone (TRH) administration resulted in a marked increase in serum TSH concentrations. Triiodothyronine, thyroxine and dexamethasone administration lowered the serum TSH concentration. There was diurnal variation in TSH secretion. Growth hormone (GH) and prolactin responses to provocative stimuli and 24 h secretion patterns were normal. FSH and LH secretion was normal for age and stage of sexual development. The 24 h cortisol pattern demonstrated normal episodic secretion, although the mean 24 h concentration was high (10.5 microgram/dl). These data suggest that this patient's TSH hypersecretion is due to partial resistance of the thyrotrophs to the inhibitory action of thyroid hormone.