Seasonal Variation in Testosterone Levels
Yes, testosterone levels do decrease during winter months, with the lowest levels typically occurring in June-August (summer in the Northern Hemisphere) and peak levels in October-November and December, representing approximately a 31% difference between seasonal extremes.
Evidence for Seasonal Testosterone Variation
The most robust evidence comes from a large cross-sectional study of 1,548 men in northern Norway, which demonstrated a bimodal seasonal pattern in total testosterone with a small peak in February, nadir in June, and more prominent peak in October-November 1. Free testosterone showed a significant seasonal pattern with peak levels in December and lowest levels in August, with these patterns persisting after adjusting for age and waist-to-hip ratio 1.
The magnitude of seasonal variation is clinically significant, with a 31% difference between the lowest and highest monthly mean levels of free testosterone 1. Importantly, the lowest testosterone levels occurred during months with the highest temperatures and longest daylight hours, not during winter 1.
Physiological Mechanisms
The seasonal variation appears inversely related to temperature and daylight exposure 1. A modeling study of 859 hypogonadal men confirmed that testosterone levels were 19.3 ng/dL (8.1%) higher during winter and spring compared to summer and fall 2. This circannual rhythm exists independently of the well-established circadian rhythm of testosterone 2.
High ambient temperatures may disrupt the circadian rhythm of reproductive hormones and alter testicular steroidogenesis gene expression 3. Cold stress may have an inhibiting effect on sexual behavior and testosterone levels, though the relationship is complex 4.
Clinical Implications
When evaluating testosterone levels clinically, the season of measurement should be considered 4. A prospective study found significant differences in testosterone levels, sexual function scores, and frequency of sexual thoughts between winter (January-February) and summer (July-August) measurements 4. Although testosterone levels remained within normal limits in both seasons, winter levels were consistently lower than summer levels 4.
The seasonal variation in testosterone parallels changes in waist-to-hip ratio, which shows highest values during summer and is inversely related to testosterone levels 1. This suggests metabolic factors may contribute to the seasonal pattern.
Important Caveats
The direction of causality requires further investigation through prospective studies 1. Multiple factors beyond season influence testosterone levels, including body weight, nutritional intake (particularly zinc, magnesium, vitamin D, and polyphenols), sleep quality and quantity, and stress levels 5. These confounding variables must be considered when interpreting seasonal testosterone measurements in clinical practice 5.
For diagnostic purposes, the American Diabetes Association recommends measuring morning serum total testosterone using an accurate assay, with repeat testing to confirm low levels due to assay variability 6. The seasonal component adds another layer of variability that clinicians should recognize when making treatment decisions 4.