We studied the thyrotropin response to protirelin challenge (200 micrograms intravenously) at 8 am and at 11 pm after a minimum washout period of 10 days in 29 euthyroid inpatients who met DSM-III-R criteria for major depressive episode and 20 normal volunteer controls. The maximum increment in thyrotropin above baseline (delta thyrotropin) was significantly greater at 11 pm than at 8 am both in patients and in controls. However, the difference between 11 pm delta thyrotropin and 8 am delta thyrotropin (delta delta thyrotropin) was significantly lower in patients than in controls. The lower delta delta thyrotropin found in patients could not be explained by differences in age, body weight, sex, or thyroid functioning. In the overall population, delta delta thyrotropin correlated with circadian variables (ie, mesor and amplitude). With the use of a criterion of less than 3 mU/L to define a blunted delta delta thyrotropin, the diagnostic sensitivity was 89% and the specificity was 95%. We suggest that delta delta thyrotropin has the advantage of taking into account chronobiologic influences in the interpretation of the protirelin/thyrotropin challenge, and this may explain the improved diagnostic value derived from this measure in the diagnosis of major depressive episode.