An analysis of millions of blood tests has shown a whole host of human hormones that fall into clear seasonal patterns, although these changes are small.
Hormones in the pituitary gland, which help control reproduction, metabolism, stress and breastfeeding, have been shown to reach their peak in late summer.
The peripheral organs under the control of the pituitary gland, such as those that produce our sex hormones or thyroid hormone, also showed seasonality. Instead of reaching their peak in summer, however, these hormones hit the ground running in winter.
Testosterone, estradiol and progesterone, for example, peaked in late winter or spring.
The findings provide the strongest evidence to date that humans have an internal seasonal clock, which somehow affects our hormones in a way that aligns with the seasons.
“Along with a long history of studies of a winter-spring peak in human function and growth, hormonal seasonality indicates that, like other animals, humans may have a peak physiological season for basic biological functions,” the authors write. .
The basic mechanism that drives this circanual clock is still unknown, but the authors suggest that there is a natural feedback loop throughout the year between the pituitary gland and the peripheral glands in the body.
Pituitary hormones, which are uniquely regulated in sunlight, could nourish these other organs over the course of a year, allowing them to grow in functional mass in a way that aligns with the seasons.
“Thus, people can present seasonal set points coordinated with a winter-spring peak in the axes of growth, stress, metabolism and reproduction,” the authors write.
As the paper mentions, it is not much different from what we find in other mammals, where fluctuations in certain hormones lead to seasonal changes in the reproduction, activity, growth, pigmentation or migration of an animal.
Mammals, such as arctic reindeer, for example, experience a decrease in a hormone called leptin as winter days become shorter, and this helps lower energy consumption, lower body temperature, and inhibit their ability to reproduce.
Even primates closer to the equator are sensitive to subtle seasonal changes. For example, Rhesus macaques ovulate much more during the post-monsoon season, so that their offspring are born just before the monsoons strike in the summer.
It remains unclear whether or not human hormones fluctuate with the seasons.
Most of the data sets that have been analyzed so far are not very large and do not cover all human hormones, which makes the conclusions very challenging. The studies examined either only human sex hormones or focused on stress and metabolic hormones. The results were also quite varied and inconsistent.
While some studies on human sex hormones suggest that seasonal changes should be considered, other studies conclude that seasons are a source of variable importance.
Meanwhile, research on salivary cortisol levels – also known as the stress hormone – has found that there is some seasonal variability, and a large data study of thyroid stimulating hormone found higher levels of this hormone in summer and winter.
The new research is the largest in the group and includes a massive dataset of Israeli medical records covering 46 million person-years. It also analyzes all human hormones.
Controlling the changes over a single day, the authors found that humans show seasonal patterns of their hormone levels, although not as strong as other mammals.
The physiological effects of these hormonal changes are not yet clear, but some of the changes in thyroid hormone, T3, and stress hormone, cortisol, align with previous findings.
For example, thyroid hormone, which has been shown to peak in winter, has been linked to heat generation. The seasonal peak of cortisol, which was found to peak in February, is also in agreement with previous studies covering the northern and southern hemispheres.
Seasonal changes are small, but, as the authors point out, from a clinical perspective, “even a small systematic effect can cause misdiagnosis if normal intervals are not adapted to the seasons, with the costs associated with additional testing and treatment.”
In order to further verify the results, more studies will have to be done on a similar scale in different parts of the world. But the results suggest that we are not that different from other mammals after all.
If our hormones really drop and flow with the seasons, even a little, it could be important for our health that we know.
The study was published in PNAS.