Bioavailable testosterone levels, which peak in the early morning, represent the amount of free testosterone and the testosterone bound to albumin (but not SHBG). Unlike the over 50% of testosterone bound to SHBG which is strongly attached, testosterone bound to albumin is weakly attached and easily able to detach and become free testosterone. It is called “bioavailable” because it is easily “available” to be used by cells. All three types of testosterone (Total, Bioavailable and Free) decline with age in both men and women after peaking in the late 20s. Because SHBG binds up more testosterone as we age, bioavailable testosterone levels decline even more than total testosterone levels. This is of concern because low testosterone levels are associated with heart disease risk factors. It is useful to test free testosterone levels in men and women who have symptoms of low testosterone but have normal total testosterone levels and can also be used to identify Andropause (the male equivalent to the female menopause) which is a collection of symptoms including fatigue and decreased libido in middle-aged men that is due to the age-related testosterone decline.

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