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Because a man's serum testosterone level varies diurnally, guidelines generally advise measuring it in early morning, when it tends to be highest. However, diurnal variation becomes less prominent as men age. In this study, U.S. researchers examined initial serum testosterone levels measured in 2600 men who attended an erectile dysfunction clinic. Results were analyzed according to patient age and according to the time blood was drawn (between 7 am and 2 pm; too few samples were drawn after 2 pm for accurate analysis).
In men younger than 45, diurnal variation was confirmed; mean levels were ≈600 ng/dL at 7 am, 500 ng/dL at 10 am, and 400 to 450 ng/dL by 2 pm. However, within each 5-year age grouping for older men (age, ≥45), mean serum test…