SUMMARYIn a case of subacute de Quervain thyroiditis characterized by a period of pronouced hypermetabolism followed by hypometabolism, levels of T4, T3 and the TSH response to TRH were repeatedly measured.As expected, the response to TRH was absent or low during the hypermetabolic phase, high during the hypometabolic phase and returned to normal with recovery. However, at the beginning and at the end of the hypometabolic phase, while circulating levels of T3 and T4 both lay in the subnormal range, two periods of inadequate response to TRH could be demonstrated. During the first period, the response to TRH was zero or very small; during the second period, stimulation by TRH was exaggerated.The response to TRH thus depends not only on the level of circulating thyroid hormones but on other factors such as the previous state of pituitary stimulation or inhibition.