Absent Diurnal Variation in Serum Testosterone in Young Men With Testosterone Deficiency.

INTRODUCTION: Although healthy young men demonstrate diurnal pattern of serum testosterone (T), minimal information is available regarding diurnal variation in young men with T deficiency.

METHODS: Blood samples were obtained over a 24-hour period at 08:00, 11:00, 14:00, 17:00, 20:00, and 08:00 the following morning. Men were categorized as normal or low T if serum T > 300ng/dL or >300ng/dL at 08:00, respectively.

RESULTS: Twenty-one volunteers (mean age 31.7y, range 18-49) were studied. Eleven had normal T and ten had low T, all with normal LH concentrations. The low T group was older (mean 33.4y vs 30.1y), with higher BMI (mean 32.6kg/m 2 vs 27.5kg/m 2); these differences were not significant. Highest overall mean T concentration was observed at 08:00 and lowest at 14:00. Mean T levels in the normal group declined between 08:00 and 14:00 from 423ng/dL to 358ng/dL (15% decline; p=0.0003). Mean T for the low T group was 228ng/dL at 08:00 and 218ng/dL at 14:00 (4% decline; p=0.54). Calculated free T paralleled total T, with a decline of 14% (p<0.001) for the normal T group and 5% (p=0.52) for the low T group. Two of eleven men in the normal group did not demonstrate diurnal variation. No individual with baseline T>400ng/dL demonstrated T <300ng/dL at any time point.

CONCLUSION: Men with low T concentrations failed to demonstrate diurnal variation with 24h blood sampling. We speculate that similar central mechanisms may be involved in the pathophysiology leading to secondary T deficiency as well as the loss of circadian rhythmicity.