1. The total serum thyroxine, tri-iodothyronine resin uptake, total plasma protein concentration and the free thyroxine index (FTI) were determined repeatedly, at 07.15, 13.00 and 22.30 hours over 4 days, in six healthy young men. 2. There was a significant diurnal variation in the total serum thyroxine concentration but this reflected changes in the binding capacity of serum proteins and in the total plasma protein concentration which could be explained by changes of posture. The FTI, and presumably therefore the free thyroxine concentration, varied very little with the time of day. 3. The FTI varied significantly from day to day in three of the six subjects, presumably as a result of changes in thyroxine secretion because the serum binding capacity did not vary. 4. The subjects' sleep at night was assessed by electro-encephalogram. On days when the FTI was highest for a particular subject his sleep was more fragmented by spontaneous awakenings, the amount of rapid-eye-movement sleep was reduced and that of delta-wave sleep was increased, implying that variations in thyroid function over a period of a few days in healthy subjects can be of physiological significance. The cause of these variations is uncertain.
1. Daily fluctuations in the plasma concentrations of thyroid stimulating hormone (TSH) and growth hormone (GH) have been studied in six normal males by using a 24 h continuous blood-sampling technique and a highly sensitive and specific radioimmunoassay for TSH. 2. In all subjects the plasma concentrations of the two hormones were increased at night. The TSH concentrations rose approx. 2 h before the onset of sleep but the peak concentration coincided with the GH peak 2 h after the commencement of sleep. In three subjects who slept in the afternoon, additional peaks of GH activity were found and in two cases peaks of TSH activity also occurred. 3. The mean data from all subjects showed a significant circadian variation of TSH secretion with a maximum between 2 a.m. and 4 a.m. and a minimum between 6 p.m. and 8 p.m.