The association of time of day and serum testosterone concentration in a large screening population

OBJECTIVE: To assess testosterone patterns during clinical hours in a large population of men participating in a national screening programme for prostate cancer, as the effect of time of day on serum testosterone concentration is unclear and largely reported in small studies. SUBJECTS AND METHODS: Testosterone levels were measured in 3006 men attending the national Prostate Cancer Awareness Week screening programme. Blood samples were obtained between 06.00 and 18.00 hours, whenever men presented for screening. All men completed questionnaires on age, comorbidities, height and weight. Testosterone levels were compared based on four periods, i.e. T1, 06.00-10.00 (632 men), T2, 10.00-12.00 (812), T3, 12.00-14.00 (388), and T4, 14.00-18.00 hours (1174). RESULTS: The mean (sd, range) age of the men was 60.3 (9.9, 40-94) years and the mean (sd) testosterone level was 415.2 (190.9) ng/dL. There was no change in mean (sd) testosterone levels over T1, T2 and T3, at 444.9 (206.2), 433.5 (195.8) and 434.4 (181.2) ng/dL, respectively, but levels at T4, at 380.4 (176.4) ng/dL, were lower by approximately 13% (P < 0.05). Advancing age, diabetes and obesity were associated with lower testosterone levels. The percentage of men with biochemical hypogonadism (<300 ng/dL) did not change across all four periods. CONCLUSIONS: Testosterone levels in older men are stable throughout the morning and early afternoon, declining only modestly thereafter. Further case-controlled studies are needed to confirm these findings.