A review of millions of blood tests has shown that a slew of human hormones fall in distinct seasonal patterns, although these changes are small in size.
Pituitary hormones, which help control reproduction, metabolism, stress, and lactation, were usually found to peak in late summer.
Peripheral organs under control of the pituitary gland, such as those that produce our sex hormones or the thyroid hormone, also showed seasonality. However, instead of peaking in the summer, these hormones kick in in the winter.
For example, testosterone, estradiol and progesterone peaked in late winter or spring.
The findings provide the strongest evidence yet that humans have an internal seasonal clock, which somehow affects our hormones in a way that matches the seasons.
“Along with a long history of studies of winter-spring peak in human function and growth, the seasonality of the hormone indicates that, like other animals, humans can have a physiological peak season for basic biological functions,” the authors write.
The underlying mechanism driving this circular clock is still unknown, but the authors suggest that there is a one-year natural feedback circuit between the pituitary and peripheral glands in the body.
The pituitary hormones, which are uniquely tuned to sunlight, could feed these other organs over the course of a year, allowing them to grow in functional mass in a way that matches the seasons.
“Humans may thus exhibit coordinated seasonal set points with a winter-spring peak in the axes of growth, stress, metabolism, and reproduction,” the authors write.
As the article notes, it is not much different from what we find in other mammals, where fluctuations in certain hormones lead to seasonal changes in an animal’s reproduction, activity, growth, pigmentation or migration.
Mammals such as arctic reindeer, for example, show a decrease in the hormone leptin when the winter days become shortest, and this helps lower their energy expenditure, lower their body temperature and diminish their ability to reproduce.
Even primates closer to the equator show sensitivity to subtle seasonal changes. Rhesus macaques, for example, ovulate significantly more during the post-monsoon season, so their offspring are born just before monsoon hits in summer.
Whether human hormones also fluctuate with the seasons remains unclear.
Most of the datasets analyzed so far are not very large and do not include all human hormones, which makes drawing conclusions very difficult. Studies have either examined human sex hormones alone, or they have 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 an insignificant source of variability.
Meanwhile, research on salivary cortisol levels – also known as the stress hormone – shows that there is some seasonal variability, and a big data study of the thyroid-stimulating hormone found higher levels of this hormone in summer and winter.
The new research is the largest of the lot and includes a huge dataset of Israeli health records spanning 46 million person years. It also analyzes all human hormones.
Checking for changes over one day, the authors 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 still unclear, but some 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, has been linked to thermogeneration. The seasonal timing of cortisol, which peaked in February, is also consistent with previous studies in the Northern and Southern Hemispheres.
The seasonal changes are small in magnitude, but as the authors point out, from a clinical perspective, “even a small systematic effect can cause a misdiagnosis if the normal range is not seasonally adjusted, with the associated costs of additional testing and treatment. “
More studies will need to be done on the same scale and in different parts of the world to further verify the results. But the findings suggest that we’re not that different from other mammals after all.
If our hormones really do move with the seasons and move with the seasons, even if just a little bit, it can be important to our health that we know.
The study is published in PNAS.