Psychiatric illness is a cause of "euthyroid sick syndrome" (ESS), defined as abnormal concentrations of circulating iodothyronines in euthyroid subjects with nonthyroidal illness (NTI). We describe a prospective study of 150 consecutive psychiatric admissions studied by endocrine and psychologic techniques. Based on 150 admission blood samples, we found a 7% incidence of ESS and with serial samples (74 patients) the incidence was 27%, demonstrating that ESS can develop after hospital admission. Of the 20 patients with ESS, 11 had elevation of both serum total T4 concentrations (T4) and free thyroxine index (FTI) while their serum total T3 concentrations (T3) remained normal; 5 had elevation of FTI without elevation of T4 or T3; and 4 had low T4 and low FTI and normal TSH. In 2 of the 4 patients in the last category, the T3 was also low. The free T3 index (FT3I) was normal in all but 1 patient who had low FT3I and FTI, low T4 and T3, and normal TSH. The serum thyroid hormone abnormalities were transient in the ESS patients during the 10 day period with 2 exceptions; 1 patient had persistently elevated T4 and FTI with normal T3 and FT3I values while another patient had persistently depressed T4 and FTI without abnormality of FT3I or TSH. Multivariate statistical analysis demonstrated a difference (P < .06) in the psychologic attributes of somatic and autonomic symptoms in ESS patients compared to controls. We conclude that ESS is as common amongst psychiatric admissions as in general hospital patients previously studied and that blood thyroid function tests should be interpreted cautiously in all hospitalized patients.