Absent Diurnal Variation in Serum Testosterone in Young Men with Testosterone Deficiency

Although healthy young men demonstrate a diurnal pattern of serum testosterone, minimal information is available on diurnal variation in young men with testosterone deficiency.

Blood samples were obtained during a 24-hour period at 8 and 11 a.m., 2, 5 and 8 p.m., and 8 a.m. the following morning. Men were categorized with normal or low testosterone if serum testosterone was greater than 300 ng/dl or less than 300 ng/dl at 8 a.m., respectively.

We studied 21 volunteers with a mean age of 31.7 years (range 18 to 49). Testosterone was normal in 11 men and low in 10 and all had a normal luteinizing hormone concentration. The low testosterone group was older (mean age 33.4 vs 30.1 years) with a higher body mass index (mean 32.6 vs 27.5 kg/m²) but the differences were not significant. The highest and lowest overall mean testosterone concentrations were observed at 8 a.m. and 2 p.m., respectively. Mean testosterone levels in the normal group declined between 8 a.m. and 2 p.m. from 423 to 358 ng/dl, representing a 15% decrease (p=0.0003). Mean testosterone in the low testosterone group was 228 ng/dl at 8 a.m. and 218 ng/dl at 2 p.m., representing a 4% decline (p=0.54). Calculated free testosterone paralleled total testosterone with a 14% decrease in the normal testosterone group (p <0.001) and a 5% decrease in the low testosterone group (p=0.52). Two of 11 men in the normal group showed no diurnal variation. No subject with baseline testosterone greater than 400 ng/dl had testosterone less than 300 ng/dl at any time point.

Men with low testosterone failed to show diurnal variation on 24-hour blood sampling. We speculate that similar central mechanisms may be involved in the pathophysiology leading to secondary testosterone deficiency as well as the loss of circadian rhythms.