Circadian Variation in Testosterone, Sex Hormone‐Binding Globulin, and Calculated Non‐Sex Hormone‐Binding Globulin Bound Testosterone in Healthy Young and Elderly Men

The circadian pattern in levels of serum total testosterone (T) in men becomes blunted with normal aging. However, because T not bound to sex hormone-binding globulin (non-SHBG-T) is felt to be a better representative of biologically available T than is total T, the possibility of a 24-h variation in non-SHBG-T in young men and the possibility that aging is associated with a blunting of that rhythm were investigated. Hourly blood samples were drawn on 10 normal young men (mean age 27.3 years) and 10 normal elderly men (mean age 70.7 years) over a 24-h period and the serum was assayed for total T, sex hormone-binding globulin (SHBG), and total protein; non-SHBG-T was calculated. SHBG was determined by radioimmunoassay as well as by a steroid-binding assay. Young men had a significantly higher (p less than 0.05) mean 24-h level of non-SHBG-T (1.91 +/- 0.62 nm/l) than did the elderly men (0.86 +/- 0.01 nM/l). Also, each young man showed a significant circadian rhythm in non-SHBG-T, with a group mean daily variation of 1.42 +/- 0.38 nM/l. In contrast, only 60% of the elderly men demonstrated a significant circadian rhythm in non-SHBG-T, and the group mean rhythm was blunted (maximum excursion 0.38 +/- 0.07 nM/l) compared with that of the young men. SHBG and total protein levels demonstrated similar 24-h variations in the two age groups. It was concluded that non-SHBG-T serum levels, similar to serum total T levels, demonstrate a circadian pattern in young men and this circadian rhythmicity becomes blunted with normal aging.