In severely ill patients of an intensive care unit overt peripheral thyroid hormone deficiency was noted in 22 of 33 subjects. The TRH-test was performed in 7 of these 22 patients and was negative in all. Thus, the laboratory data suggest secondary hypothyroidism. The laboratory diagnosis, however, could not be supported by clinical signs probably due to the short period of observation and to the strongly elevated body temperature of the respective patients. The pathogenetic mechanism is not clear. However, all patients developing secondary hypothyroidism were treated with dopamine and/or glucocorticoids, compounds known to inhibit pituitary TSH release and most of the patients had septic fever. A persistently "hypothyroid state" (total T4: 23 +/- 15 nmol/l, free T4: 6.1 +/- 3.2 pmol/l, total T3: 0.28 +/- 0.22 nmol/l, S.D.) is associated with a very poor prognosis. In view of the negative effects of a hypothyroid state, substitution of thyroid hormones must be considered in this situation.