Abstract
In the present study, we investigated if there are any diurnal variations in serum thyroid stimulating hormone (TSH) levels in normal subjects and any alterations in serum TSH, free thyroxine (fT4), and free triiodothyronine (fT3) levels with aging. Our results showed the presence of a circadian rhythm in serum TSH levels in normal subjects (n = 11; 5 males, 6 females; age range 19–46) with no significant changes in fT3 and fT4, indicating that the regulation of TSH is central rather than peripheral. In addition, we examined samples from 1,571 euthyroid outpatients (513 males, 1,058 females; age range 10–90) over a 6-month period. Serum TSH levels in age groups 50–59, 60–69, 70–79, and 80–89 were found to be progressively higher than those in the age group 10–19 with no significant differences between male and female populations. These findings suggest the presence of an alteration in TSH secretion with aging.
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