Serum testosterone and its relation to bone mineral density and body composition in normal males

Bone mineral density (BMD) declines with age in both men and women, predisposing the elderly to osteoporosis and fractures. Although there are extensive data about post-menopausal osteoporosis, there is relatively little information concerning the decrease in BMD with age in normal men, particularly the contribution of declining gonadal function with age to BMD. In the present study, we investigated the effect of age on the pituitary-gonadal axis in normal males and its relation to BMD and body composition.

Ninety healthy Thai males in the Bangkok Metropolitan area without a history of smoking or significant alcohol consumption were studied.

Serum testosterone (T), free testosterone (FT), LH and FSH were measured by radioimmunoassay in fasting blood samples obtained in the morning between 0600 and 1000 h. BMD at anteroposterior L2-L4, lateral L2-L4, femoral neck, femoral trochanter and Ward's triangle were determined by dual-energy X-ray absorptiometry.

There were significant declines with age in BMD at lateral L2-L4 (r = -0.37, P < 0.001), femoral neck (r = -0.49, P < 0.0001), Ward's triangle (r = -0.54, P < 0.0001) but not at anteroposterior L2-L4 or femoral trochanter. Serum FT (r = -0.56, P < 0.0001) but not T (r = -0.19, P = 0.07) decreased with age. Serum LH (r = 0.27, P < 0.001) and FSH (r = 0.4, P < 0.0001) increased with age suggesting a defect in gonadal androgen synthesis or possibly a secretion of bioinactive LH. Serum FT concentrations were significantly correlated to lateral L2-L4 (r = 0.27, P < 0.05), femoral neck (r = 0.48, P < 0.0001) and Ward's triangle (r = 0.50, P < 0.0001) BMD. After controlling for age, declining FT with age was still associated with a decrease in BMD in femoral neck (P < 0.05) and Ward's triangle (P < 0.05) but not in lateral L2-L4. The proportion of body fat increased with age (r = 0.3, P < 0.01). Decreased serum T, but not FT, was associated with increased body fat after age was taken into account (P < 0.0001).

There is a decline in serum free testosterone together with increases in LH and FSH with age in healthy males. The decrease in serum free testosterone is partially associated with the age-related decline in bone mineral density added to the effect of age at the femoral neck and Ward's triangle. Testosterone but not free testosterone is associated with age-related increase in body fat.