Circannual rhythms of plasma growth hormone, thyrotropin and thyroid hormones in prepuberty.

To clarify the occurrence of circannual GH, TSH, T4 and T3 rhythms in prepuberty we have been studying, for a four year period, 150 healthy subjects, aged 6-10, by cross-sectional design. Plasma samples were taken at 0800 h and all hormones were measured by RIA. The occurrence of any circannual rhythm was statistically investigated by the cosinor method. A significant rhythm was validated (P = 0.02) only in TSH secretion, with annual crest time in December; GH, T4, T3 did not show a circannual rhythm. Our results seem to strengthen the hypothesis that the thyroid hormones, at least before puberty, do not play an important role in the regulation of circannual TSH periodicity.