Purpose: We investigated whether serum testosterone levels vary monthly in middle-aged men.
Materials and methods: A total of 8,367 middle-aged men who participated in a police officer urological health screening program at National Police Hospital were included. The participants were applicants. Total testosterone was measured early in the morning. Additionally, data on age, waist circumference, blood pressure and levels of fasting blood sugar, triglycerides and high density lipoprotein were collected as confounders. We also collected the monthly temperature and daylight duration in Seoul. We compared the mean testosterone level in each month before and after adjusting for confounders.
Results: Testosterone showed a monthly pattern with a nadir in May (4.4 ng/ml) and a peak in January (6.0 ng/ml). The mean testosterone level differed significantly across the months (p <0.001). After adjusting for confounders, testosterone still showed a monthly pattern with a nadir in May (4.5 ng/ml) and a peak in January (6.1 ng/ml). The trendlines of testosterone and the daylight duration in each month suggested a negative relationship, and the trendlines of testosterone and outdoor temperature suggested a negative relationship.
Conclusions: The serum testosterone levels varied monthly, and this variation seems to be related to outdoor temperature and daylight duration. Our results warrant further investigation.
Keywords: hypogonadism; sunlight.; temperature; testosterone.