Hypogonadism, aka testosterone deficiency or low-T, is primarily diagnosed by low total testosterone levels. However, more and more research is showing that free testosterone, which is the active fraction of total circulating testosterone, is independently associated with important health outcomes.
Levels of free testosterone decline more steeply than total testosterone as menage.[1-7 In many cases, total testosterone levels can be relatively high, but free testosterone low. Therefore it is important to assess both total and free testosterone levels in order to get a clear picture of the androgen status.
A recent study specifically assessed if baseline testosterone (total and free) levels predict muscle loss in middle-aged and elderly Japanese men over a 10 year period.