Altered TSH and its Circadian Rhythm

A circadian rhythm of serum thyrotropin (TSH) has been demonstrated by measurements performed on the majority of blood from 6 healthy young adults, 3 males and 3 females. The serum TSH level decreased from 8 am to 2 pm. The difference between the morning and midday levels was very visible. There was no correlation between meals and TSH level variations. The circadian rhythm of serum TSH is not only of physiological interest but also has practical implications where collecting blood for TSH determination is best done at 11:00 a.m. when the TSH level has fallen to the daytime amount. The mechanism of decreasing TSH levels in the blood within 24 hours is still not fully studied.