Testosterone is a steroid hormone produced primarily by the testes in males and, to a lesser extent, by the ovaries and adrenal glands in females. As the main androgen, it drives the development of male secondary sexual characteristics, such as the deepening of the voice and the growth of facial hair. In both sexes, testosterone is involved in numerous systemic processes, including maintaining muscle mass, bone density, regulating metabolism, and supporting mood and energy levels. The hormone circulates throughout the bloodstream, but its availability to tissues depends on the form it takes. The body’s total supply is composed of different fractions, each with a distinct biological status.
Total Testosterone: The Overall Measurement
Total Testosterone is the standard number reported in routine blood work, representing the cumulative measure of all testosterone molecules circulating in the bloodstream. This single value is the baseline measurement used by clinicians to diagnose hormone imbalance, encompassing every molecule, whether usable or locked away by transport proteins. While necessary for diagnosis, this measurement does not fully reflect the amount of hormone available to perform biological functions. The total value includes a small biologically active portion and a much larger inactive portion. An individual can have a seemingly normal Total Testosterone level yet still experience symptoms of low hormone activity if the distribution between these forms is unbalanced.
Bound Testosterone: The Inactive Portion
Bound Testosterone is the vast majority of the hormone measured in the blood, physically attached to carrier proteins. This binding renders the hormone inactive, preventing it from entering cells and interacting with receptors. Approximately 97% to 98% of circulating testosterone is in this bound state, acting as a reservoir of the hormone in the bloodstream.
The primary carrier protein is Sex Hormone-Binding Globulin (SHBG), a glycoprotein synthesized mainly by the liver. Testosterone molecules bound to SHBG are held tightly, making this fraction biologically inert. SHBG acts as a regulator, managing how much testosterone is available for immediate use by tissues.
A smaller portion of testosterone is bound more loosely to albumin. Because the bond with albumin is weak, this fraction can easily detach from the protein as it moves through the capillaries, allowing it to become available to tissues. Historically, all testosterone not bound to SHBG—both the albumin-bound and the completely unbound portions—has been collectively referred to as “bioavailable testosterone.”
Free Testosterone: The Biologically Active Form
Free Testosterone is the fraction of the hormone that is not bound to any protein. This unbound portion is small, typically representing only about 1% to 3% of the Total Testosterone measurement. Despite its small quantity, Free Testosterone is considered the biologically active form that drives the hormone’s effects.
Because it is not attached to any carrier, Free Testosterone can readily leave the bloodstream and pass through the cell membranes of target tissues, such as muscle and bone. Once inside the cell, it is able to bind to androgen receptors, initiating the processes responsible for muscle growth, energy, mood regulation, and sexual function. Measuring Free Testosterone is often done when SHBG levels are abnormal, as it provides a more accurate picture of the hormone the body can utilize. This direct measure offers a better understanding of an individual’s true hormonal status than the Total Testosterone number alone.