Testosterone levels can fluctuate over the course of the day, so medical guidelines recommend that men get blood drawn for hormone tests in the morning, when testosterone is highest. But morning measurements may be less important for older men, according to a study in the Journal of Urology.
Researchers looked at testosterone levels measured in 2,600 men who went to an erectile dysfunction clinic. In men younger than 45, testosterone was highest at 7 a.m. but as much as 33% lower by 2 p.m. In contrast, the levels in older men varied very little.
However, testosterone levels can vary for reasons other than time of day, making it uncertain that a single test result truly reflects the level of biologically active hormone in a man’s system. For example, having a chronic illness or being overweight can suppress testosterone levels. Borderline-low test results can be especially hard to interpret.
Based on a single test, it’s possible a man could end up being incorrectly diagnosed as having abnormally low testosterone and possibly offered testosterone supplementation he doesn’t really need. Having testosterone measured on a different day may be valuable in such cases. It’s also important for the doctor to assess whether a man has key symptoms of testosterone deficiency, such as low libido, erectile dysfunction, low sperm count, loss of body hair, and hot flashes.