A 48-h dopamine (DA) infusion (5-7.5 microgram/kg . min) given to six healthy euthyroid males resulted in a suppression of thyroidal iodine release and serum TSH by 44 +/- 3% (P less than 0.01), serum T3 by 9 +/- 2% (P less than 0.01), and serum T4 by 5 +/- 1% (P less than 0.05) below baseline levels, without a significant change in serum rT3 levels. In critically ill patients receiving DA (2-21 microgram/kg . min) for treatment of shock, serum TSH values and T4 production rates were decreased 60% and 56%, respectively, below the respective levels observed in non-DA-treated patients (P less than 0.01). Serial serum samples collected before and during DA therapy revealed a decrease of 52% in TSH (P less than 0.005) and 30% in T4 (P less than 0.05). The finding of a normal serum TSH value during DA theray in a critically ill patient with primary hypothyroidism emphasized the inhibitory potential of DA on TSH secretion. These findings indicate that the prolonged administration of pharmcological doses of DA significantly reduced serum TSH levels and thyroid hormone secretion in normal and critically ill patients, most likely by a direct inhibition of pituitary TSH with a secondary effect on thyroid gland secretion. Therefore, DA therapy probably prolongs and aggravates the low T4 state in critical illness.