Free testosterone is the portion or fraction of circulating testosterone that isn’t bound to other proteins. Since bound testosterone is considered unable to practice its effect on target tissues, free testosterone is also considered to be the active form of testosterone.
Free testosterone is measured in picograms per milliliter (pg/mL) or nanomoles per liter (nmol/L).
Normal levels of free testosterone indicate that your hormones and liver function are in balance.
Measuring free testosterone can be relevant for several reasons, where symptoms of low testosterone levels might be the most common, since free testosterone measurements can help to confirm a diagnosis of low testosterone.
It can also help in the investigation of male infertility, since low testosterone levels in men can contribute to infertility. When treating with testosterone replacement therapy, measuring free testosterone can help monitor progress as well as evaluating the effectiveness of treatment.
Low levels of free testosterone mean that the production of testosterone is low, or that more of the circulating testosterone is bound to proteins than what is normal. Age is probably the most common underlying cause behind low levels, since increasing age is directly associated with a naturally decreased free testosterone level.
This does not necessarily mean that every man with decreased levels at a high age suffers from testosterone deficiency. In order to be considered suffering from a deficiency, you need low levels in blood as well as symptoms associated with deficiency, also known as hypogonadism. Ot
High levels of free testosterone are not very common, and can either mean that there is an overproduction of testosterone, or that the affinity to or amount of the proteins that bind to testosterone are lower than normal. The most common reason for such an increase is age (although still rare), caused by a decreased affinity to the proteins it binds to. Less common causes that result in increased testosterone production are for example genetic disorders that affect the adrenal glands or tumors in the adrenal glands or testicles.