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A circadian rhythm of serum thyrotropin (TSH) has been demonstrated by measurements performed on half-hourly blood samples collected over a 24-hour period from 8 healthy young men and 5 healthy young women. The serum TSH level decreased from 8 a.m. to 11 a.m. There was a low level during the day-time with a mean serum TSH value of 0.85 μU/ml. The level increased in the evening between 8 p.m. and 11 p.m., and a higher TSH level was maintained during the night, with a mean value of 1.34 Ü/ml. The difference between day and night level was highly significant. There was no relation between the meals and the variation of the TSH level. The rise of serum TSH preceded the onset of sleep. The circadian rhythm of serum TSH is not only of physiological interest but has also the practical consequence that blood sample collection for TSH determinations is best performed at 11 a.m., when the TSH level has fallen to the day-time level.
J. Weeke (Mon,) studied this question.