To evaluate the existence of circadian variations in thyroid hormone and TSH levels, blood was drawn every 20 min for 24 h in four and for 14 h in one volunteer. Hormones were measured by sensitive radioimmunoassays.TSH: A diurnal rhythmicity could be demonstrated with peaks from 8 p. m. to 2 a. m. and a nadir from 7 a. m. to 2 p. m. Superimposed on the diurnal rhythm multiple shortlived fluctuations were observed. Thyroxine: Pooled data showed peak values from 8 a. m. to 12 a. m. and lowest levels from 11 p.m. to 3 a.m. Again, shortlived fluctuations were superimposed on the diurnal rhythm in all instances. Triiodothyronine: Hormone levels were highest from 7 a. m. to 1 p. m. and lowest from 11 p. m. to 3 a. m. Fluctuations in T3 levels were less marked than those of T4.Diurnal variations in hormone levels can be demonstrated in most, but not in all instances. Fluctuations are minor and do not exceed the normal range. Therefore these changes are of no relevance in routine testing of these hormones.There is ample evidence for the existence of diurnal secretory rhythmicity for various hormones (see Weitzman 1974 for review). Apparently, diurnal fluctua¬ tions of several hormones (GH, prolactin) are sleep-related, while no mechanism