Usefulness and limitation of bioavailable testosterone in assessment of androgenicity during the process of aging in men

Abstract

The albumin-bound testosterone diffuses easily from the systemic circulation to organs and tissues and it is now common knowledge that ‘free’ (1–2%) and albumin-bound testosterone (30–40%) should be considered as the steroid signals at work in the cell nucleus. Currently, these two androgen fractions are put together under the appellation of bioavailable testosterone while the rest of the testosterone pool is bound to sex hormone binding globulin (SHBG). In this paper, we have brought to life a method we published 25 years ago which was based on the utilization of the ammonium sulfate precipitation of SHBG in order to determine bioavailable testosterone. The method is applied to the evaluation of bioavailable testosterone in 304 healthy men. Bioavailable testosterone was measured directly by a radioimmunoassay in the supernatant of 100μl serum exposed to a 50% cold and saturated ammonium sulfate solution (100 μl). With each decade of life, we show a progressive and rather linear decrease of bioavailable testosterone with an unequivocal interindividual variability. By the age of 60, very few individuals have bioavailable testosterone values lower than 2 nmol/l. The setting up of such a threshold below which a man should be considered as hypogonadal is of paramount importance. It is currently justified, both for the security of the patients and for the evidence-based practice of physicians, and also in terms of clinical trials which will be sponsored for a few decades to assess the benefits of androgen therapy in milder cases of hypogonadism or partial androgen deficiency in men who have reached the age of 50 years or more.

Key Words:

• BIOAVAILABLE TESTOSTERONE
• THRESHOLD
• AGING
• HYPOGONADISM