A review of millions of blood tests has shown a number of human hormones that fall into clear seasonal patterns, although these changes are of minor magnitude.
Pituitary hormones, which help control reproduction, metabolism, stress, and lactation, usually peaked in late summer.
Peripheral organs under the control of the pituitary gland, such as those that produce our sex hormones or the thyroid hormone, also showed seasonality. Instead of peaking in summer, these hormones peaked in winter.
For example, testosterone, estradiol and progesterone peaked in late winter or spring.
The results provide the strongest evidence yet that humans have an internal seasonal clock that affects our hormones in a way that matches the seasons.
“Along with a long history of studies of a winter-spring peak in human function and growth, hormone seasonality suggests that humans, like other animals, may have a physiological peak season for basic biological functions,” the authors write.
The underlying mechanism that drives this circulatory clock is still unknown, but the authors suggest that there is a natural, year-long feedback loop between the pituitary and peripheral glands in the body.
The pituitary hormones, which are specifically tuned to sunlight, could nourish these other organs over the course of a year, allowing them to grow in functional mass in a way that matches the seasons.
“Humans can have coordinated seasonal target values with a winter-spring peak in the axes of growth, stress, metabolism and reproduction,” the authors write.
As noted in the paper, it’s not too different from what we find in other mammals, where fluctuations in certain hormones result in seasonal changes in an animal’s reproduction, activity, growth, pigmentation, or migration.
For example, mammals such as arctic reindeer show a decrease in the hormone leptin when the winter days are the shortest. This helps lower their energy expenditure, lower their body temperature, and inhibit their fertility.
Even primates closer to the equator are sensitive to subtle seasonal changes. For example, rhesus monkeys ovulate much more frequently in the post-monsoon season, so their offspring are born just before the monsoon in summer.
Whether or not human hormones fluctuate with the seasons or not remains unclear.
Most of the data sets that have been analyzed so far are not very large and do not cover all human hormones, which makes it very difficult to draw conclusions. Studies have either just looked at human sex hormones or have focused on stress and metabolic hormones. The results were also very different and inconsistent.
While some studies on human sex hormones suggest that seasonal changes should be considered, other studies conclude that seasons are an unimportant source of variability.
Research on salivary cortisol levels – also known as the stress hormone – shows that there is seasonal variation, and a big data study on thyroid-stimulating hormone found that hormone levels are higher in summer and winter.
The new research is the largest of the lot and includes an extensive dataset of Israeli health records spanning 46 million person-years. It also analyzes all human hormones.
The authors controlled changes in a single day and found that humans exhibit seasonal patterns in their hormone levels, although not as strongly as other mammals.
The physiological effects of these hormonal shifts are not yet clear, but some of the changes in the thyroid hormone T3 and the stress hormone cortisol are consistent with previous findings.
For example, the thyroid hormone, which peaked in winter, was bound to thermogeneration. The seasonal timing of cortisol, which peaked in February, is also in line with previous studies across the northern and southern hemispheres.
The seasonal changes are small, but as the authors point out from a clinical point of view, “even a small systematic effect can cause a misdiagnosis if the normal ranges are not adjusted to the seasons, with additional costs for additional tests and treatments.”
Further studies on a similar scale and in different parts of the world will need to be conducted to further verify the results. But the results suggest that we are not that different from other mammals after all.
If our hormones really do go up and down with the seasons, even a little, it might be important to our health that we know.
The study was published in PNAS.