Testosterone is a hormone produced primarily in the testes in men, ovaries in women, and adrenal glands in both sexes. It plays a role in various bodily functions beyond reproductive health and male characteristics. The concept of “bioavailable” testosterone explains how the body accesses and uses this hormone.
Forms of Testosterone and Bioavailability
Testosterone circulates in the bloodstream in different forms, not all of which are readily available for use by the body’s tissues. Total testosterone refers to the sum of all these forms.
Most testosterone in the blood is bound to proteins. About 60-70% of circulating testosterone is tightly bound to Sex Hormone Binding Globulin (SHBG). This strong binding renders the testosterone largely inactive, meaning it cannot easily detach from SHBG to interact with cells.
Another significant portion, about 30-40%, is loosely bound to albumin. This albumin-bound testosterone can dissociate relatively easily, making it available for cells to use. A small percentage, 1-4%, circulates as “free” testosterone, not bound to any protein.
Bioavailable testosterone is the sum of free testosterone and albumin-bound testosterone. These two forms are considered bioavailable because they can readily unbind from their carrier proteins and enter cells to exert their effects. In contrast, SHBG-bound testosterone is not considered bioavailable due to its strong attachment.
Why Bioavailable Testosterone Is Crucial
Bioavailable testosterone is the form that interacts with cell receptors throughout the body, initiating various physiological processes. Its availability directly influences numerous functions.
In men, bioavailable testosterone plays a role in maintaining muscle mass and strength. It also contributes to bone density, helping to prevent conditions like osteoporosis. Adequate levels are involved in the production of red blood cells.
Beyond these physical aspects, bioavailable testosterone affects energy levels and mood regulation. It also contributes to libido and sexual function. Its influence extends to cognitive functions, including memory and spatial ability.
Even though present in lower concentrations in women, bioavailable testosterone still contributes to several important functions. It supports bone health and plays a role in maintaining libido.
Assessing Bioavailable Testosterone Levels
Measuring testosterone levels involves specific laboratory tests that differentiate between its various forms. While total testosterone provides overall concentration, it does not indicate how much is usable by the body.
One common method involves directly measuring free testosterone in the blood. Another approach is to calculate bioavailable testosterone using a formula that considers total testosterone, albumin, and SHBG levels.
Healthcare providers might measure bioavailable testosterone when total testosterone levels seem normal but symptoms of low testosterone are present. This can occur if SHBG levels are high, leading to inactive testosterone, even if the total amount is normal. Conversely, if SHBG levels are low, a seemingly low total testosterone might still correspond to adequate bioavailable levels.
Influences on Bioavailable Testosterone
Numerous factors can influence bioavailable testosterone levels. These include natural processes, lifestyle factors, and medical conditions.
Age is a factor, as bioavailable testosterone levels naturally decline as individuals get older. This decrease often begins in men around age 30. Circadian rhythms also play a role, with levels peaking in the morning and declining throughout the day.
Acute stress can temporarily affect bioavailable testosterone levels by influencing hormone balance. Significant changes in body weight, particularly obesity, can alter SHBG levels, impacting bioavailable testosterone. Chronic illnesses, such as kidney or liver conditions, can disrupt hormone metabolism and protein binding. Certain medications, including some opioids and corticosteroids, can also influence testosterone production or its binding to proteins.