OBJECTIVE: The sick euthyroid syndrome is a poorly understood hallmark of critical illness. Dopamine is a natural catecholamine with hypophysiotrophic properties, that is used as an inotropic agent of first choice in intensive care medicine. We explored the effect of dopamine infusion (5 micrograms/kg/min) on the sick euthyroid syndrome of critically ill patients. PATIENTS AND DESIGN: In a prospective, randomized, controlled and open-labelled study of critically ill, adult polytrauma patients (n = 12), we evaluated the effect of prolonged (83-296 hours) dopamine infusion (5 micrograms/kg/min i.v.) on the thyroid axis. The effect of brief (15-21 hours) dopamine administration was documented in an additional randomized, controlled, cross-over study involving 10 patients. The median age of the studied patients was 29 (16-83) years. MEASUREMENTS: Serum TSH concentrations were measured by IRMA. The TSH profiles were obtained by blood sampling every 20 minutes for 9 hours during two consecutive nights. Serum T4, T3 and reverse T3 concentrations were measured by RIA once per study night. RESULTS: Withdrawal of prolonged dopamine infusion was found to elicit a tenfold increase of serum thyrotrophin concentrations, a 57 and 82% rise of T4 and T3 respectively, and an increase of the T3/rT3 ratio, resulting in virtual normalization of the thyroid axis within 24 hours. The brief dopamine infusion was documented to have a suppressive effect on the thyroid axis within 24 hours. CONCLUSIONS: Dopamine infusion appears to induce or aggravate the sick euthyroid syndrome in critical illness. As a consequence, the sick euthyroid syndrome of severely ill patients receiving dopamine may be not an adaptive mechanism, but a condition of iatrogenic hypothyroidism.