Older men commonly have low or low-normal total testosterone levels, but high SHBG and low free testosterone levels. Even if free testosterone levels may be a more accurate indicator of true androgen status, there are major problems with free testosterone assays.

Free testosterone assays offered by most laboratories are based on analog immunoassays, which underestimate free testosterone levels; the provided values are about 1/5 to 1/8 of free testosterone by equilibrium analysis or calculated. If free testosterone is evaluated, it should be measured using equilibrium analysis (the gold standard reference method), which is more expensive and not commonly available. The second-best option is to calculate free testosterone from total testosterone and SHBG measurements, using an algorithm.1 A good online calculator can be found at https://www.nebido.com/en/hcp/research/testosterone-tools/free-calculator.php

There are no generally accepted lower limits of normal for free testosterone. However, a free testosterone level below 220 pmol/L (64 pg/nl) may provide supportive evidence for testosterone treatment).