Seasonal Changes of Serum Gonadotropins and Testosterone in Men Revealed by a Large Data Set of Real-World Observations Over Nine Years.

Plain-language summary

This study set out to see if men's reproductive hormones follow seasonal patterns, as they do in many animals. It analyzed over 12,000 hormone tests from nearly 7,500 men aged 18 to 91, tracking testosterone, luteinizing hormone (LH, the pituitary signal that tells the testicles to make testosterone, so a lower level means less stimulation to produce it) and follicle-stimulating hormone (FSH, the signal that drives sperm production, which the body pushes higher when the testicles are underperforming) across nine years. Testosterone peaked in summer and tracked with warmer temperatures and longer daylight, while LH peaked twice, in spring and autumn, unrelated to weather. FSH showed no seasonal rhythm at all. The findings suggest that men's testosterone rises and falls with the seasons, likely responding to heat and light, while the brain's signals to the testicles keep their own separate clock.

Key findings

• Testosterone was highest in summer and tracked with warmer temperatures and longer daylight.
• LH peaked twice, in spring and autumn, and neither peak tracked the weather.
• FSH showed no seasonal pattern at all.
• Testosterone's seasonal swing did not follow LH, so the brain's signal and the testicles' output moved separately.