Recently, it has been concluded that measurement of the serum free
testosterone level is crucial for evaluating male gonadal function. However, the extent of the decline of serum free
testosterone levels with aging and the actual levels in
male infertility have not yet been clearly defined. In this study, the clinical significance of serum free
testosterone levels was evaluated in a total of 248 subjects, including 120 healthy adult males (54 males aged 20-39, 26 males aged 40-59 and 41 males aged more than 60), 94 infertile males, 28 male
hemodialysis patients, and 6 patients with
Klinefelter's syndrome. Since the serum free
testosterone levels correlate significantly with serum LH and FSH levels among 120 normal adult males, it appears that free
testosterone has a
biological action on the organ. In the subjects aged over 60, serum free
testosterone levels were significantly decreased compared with the decrease of serum total
testosterone. Thus, biologically active
androgen levels decreased with aging. Serum free
testosterone levels tended to decrease significantly from 40 years onwards. In infertile males, serum total
testosterone levels were equal to those in normal adult males, but their serum free
testosterone levels were significantly lower. This decrease of serum free
testosterone may be one of the causes of their
hypospermatogenesis. In male
hemodialysis patients, serum total and free
testosterone levels were not lower than in normal adult males. It is considered that the decline of percentage of serum free
testosterone levels in aged males and infertile males was caused by increased serum
sex hormone binding globulin (SHBG) levels. Several workers have shown that the production of SHBG is regulated by
sex steroid hormones. In this study, however, serum SHBG levels were not correlated with the E2/T ratio. We concluded that measurement of the serum free
testosterone level is of value in the endocrinological evaluation of male gonadal function.