Circannual rhythm of plasma thyrotropin in middle-aged and old euthyroid subjects.

The circannual rhythm of plasma thyrotropin (TSH) was evaluated in 8,310 euthyroid, serially independent, young, middle-aged and old men and women. A statistically significant circannual rhythm of plasma TSH was validated, by the mean group-cosinor method, in the middle-aged and old men and women (p less than 0.05), with acrophase in December, whereas the young subjects did not show any rhythm. No significant correlation was found between TSH plasma levels and free thyroxine (fT4) or ambient temperature in any group. Moreover, plasma fT4 did not show seasonal variations.