A review of millions of blood tests showed a series of human hormones that fit clear seasonal patterns, although these changes are small in magnitude.
The hormones of the pituitary gland, which help control reproduction, metabolism, stress and lactation, peaked mainly in late summer.
Peripheral organs under the control of the pituitary, such as those that produce our sex hormones or thyroid hormone, also showed seasonality. Rather than peak in summer, however, these hormones peak in winter.
Testosterone, estradiol and progesterone, for example, reached their peak in late winter or spring.
The findings provide the strongest evidence to date that humans have a seasonal internal clock, which somehow impacts our hormones in a way that aligns with the seasons.
“Along with a long history of studies on a winter / spring peak in human function and growth, the seasonality of the hormone indicates that, like other animals, humans may have a physiological peak season for basic biological functions,” write the authors .
The underlying mechanism that triggers this circus clock is still unknown, but the authors suggest that there is a one-year natural feedback loop in action between the pituitary and the body’s peripheral glands.
The pituitary hormones, which are tuned exclusively to sunlight, can feed these other organs over the course of a year, allowing them to grow in functional mass according to the seasons.
“Thus, humans can present seasonal adjustment points coordinated with a peak of winter-spring in the axes of growth, stress, metabolism and reproduction”, write the authors.
As the article mentions, 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 like the Arctic reindeer, for example, show a decrease in a hormone called leptin when winter days get shorter, and this helps to decrease energy consumption, lowering body temperature and inhibiting their ability to reproduce.
Even primates closest to the equator show sensitivity to subtle seasonal changes. For example, Rhesus monkeys ovulate significantly more during the post-monsoon season, so that their children are born just before monsoons arrive in the summer.
Whether or not human hormones also fluctuate with the seasons is still unclear.
Most of the data sets analyzed so far are not very large and do not cover all human hormones, which makes it very challenging to draw conclusions. The studies examined 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 an unimportant source of variability.
Meanwhile, research on salivary cortisol levels – also known as stress hormone – has found that there is some seasonal variability, and a large study of data on thyroid stimulating hormone found higher levels of this hormone in summer and winter.
The new survey is the largest of the lot and includes a huge set of data from Israeli health records covering 46 million person-years. It also analyzes all human hormones.
By controlling changes over the course of a single day, the authors found that humans show seasonal patterns in their hormone levels, although not as intensely as other mammals.
The physiological effects of these hormonal changes are not yet clear, but some of the changes in the thyroid hormone, T3, and the stress hormone, cortisol, align with previous findings.
For example, thyroid hormone, which peaked in winter, has been linked to thermogeneration. The seasonal cortisol weather, which peaked in February, also agrees with previous studies covering the northern and southern hemispheres.
Seasonal changes are of small magnitude, but as the authors point out, from a clinical perspective, “even a small systematic effect can cause misdiagnosis if the normal ranges are not adapted to the seasons, with the associated costs of extra testing and treatment.”
Further studies on a similarly large scale and in various parts of the world will need to be done to verify the results in more detail. But the findings suggest that we are not so different from other mammals, after all.
If our hormones actually decrease and decrease with the seasons, even a little, this can be important for our health.
The study was published in PNAS.