Human hormone seasonality

Hormones of the pituitary control the major biological functions of stress, growth, metabolism and reproduction. In animals these hormones show pronounced seasonality, with different set points for different seasons. In humans, the seasonality of these hormones has rarely been studied in a large enough dataset to discern common patterns. Here, we analyze an Israeli health record on 46 million person-years, including millions of hormone blood tests. We find clear seasonal patterns: the pituitary hormones peak in summer, whereas their downstream effector hormones are in anti-phase, and peak in winter-spring. This anti-phase is unexpected because the pituitary hormones control the downstream hormones. We show that the antiphase can arise from changes in the mass of the glands due to trophic effects of the hormones, generating a feedback circuit with a natural frequency of about a year that can entrain to the seasons. Thus, humans may show coordinated seasonal set-points with a winter-spring peak in the growth, stress adaptation, metabolism and reproduction axes.