This study was undertaken to evaluate the circadian variation of TSH in severely ill patients. Twelve patients (11 males and 1 female) admitted with severe life threatening nonthyroidal illness (NTI) and eleven normal controls (11 males) were recruited for the study. To demonstrate the nighttime TSH elevation, samples for TSH measurement were obtained on 17:00, 23:00, 01:00, and 08:00. A nocturanl TSH elevation was present in 4 out of 12 NTI patients and all control subjects. An absent nocturnal TSH elevation occurred in 3 out of 3 patients with a low TSH level (< 0.4 mU/L), in 5 out of 9 patients with a normal TSH level (0.4~4.1mU/L). No differences were found between 8 NTI patients without nocturnal TSH elevation and 4 patients with nocturnal TSH elevation in age, palsma T4, T3, and FT4 index. Nocturnal plasma cortisol levels in NTI groups were significantly higher than the controls, but the cortisol levels in NTI patients with nocturnal TSH elevation were similar to the value of NTI patients without nocturnal TSH elevation. We conclude that NTI patients are frequently associated with a decreased nocturnal TSH elevation. In addition, increased endogenous cortisol secretion in NTI patient is not likely to be responsible for the decrease in nocturnal TSH elevation (J Kor Soc Endocrinol 6:13~140, 1991).