Testosterone is an androgen hormone present in both men and women, playing a major part in maintaining bone density, muscle mass, mood, and sexual function. It is produced in the testes and adrenal glands in men, and in the ovaries and adrenal glands in women. The body moves this hormone through the bloodstream in different forms, and understanding these variations is necessary for accurately assessing hormonal health. This article focuses on “free testosterone” and why its measurement is important.
Defining Free vs. Total Testosterone
Total testosterone represents the complete sum of all the testosterone molecules circulating within the bloodstream at any given moment. This total amount includes both the portion that is attached to proteins and the small fraction that is unattached. The majority of circulating testosterone, typically around 98% or more, is bound to plasma proteins.
This bound testosterone is not immediately available for the body’s tissues to use. The primary binding agent is a protein called Sex Hormone Binding Globulin (SHBG), which holds onto testosterone tightly, rendering it biologically inactive. A secondary protein, albumin, also binds to testosterone, but this bond is much weaker, meaning the hormone can more easily break free and become available.
Free testosterone is the small percentage of the hormone that is not bound to any protein, making it the biologically active form ready to enter cells and interact with androgen receptors. This unbound fraction, which usually makes up only 2% to 3% of the total testosterone, directly influences physical and psychological processes like muscle growth, libido, and energy levels. It is often viewed as the most direct indicator of androgen activity.
Clinical Importance of Measuring Free Testosterone
The free fraction offers a more accurate picture of hormone activity than total testosterone alone. Since only free testosterone is capable of entering cells and triggering hormonal effects, its concentration best reflects the active hormone available to the body’s tissues. Measuring total testosterone sometimes provides misleading results, especially when SHBG levels are abnormal.
Conditions like aging, obesity, hypothyroidism, and liver disease can significantly change the concentration of SHBG in the blood. For instance, if SHBG levels are elevated due to a chronic condition or advancing age, more testosterone will be bound and inactive, even if the total testosterone level appears to be within the normal reference range. In such cases, measuring free testosterone reveals the true, lower level of biologically active hormone, aligning with a patient’s symptoms of deficiency.
Free testosterone measurement is a valuable diagnostic tool, particularly when total testosterone results are inconclusive or when symptoms do not correlate with the total level. It is frequently measured to diagnose conditions of androgen deficiency, such as hypogonadism in men, or conditions of androgen excess, such as Polycystic Ovary Syndrome (PCOS) in women, especially when the total testosterone level is borderline.
Interpreting Results: What High or Low Levels Mean
The implications of abnormal free testosterone levels differ significantly between men and women, requiring careful interpretation based on sex and age. In men, low free testosterone is strongly associated with symptoms of hypogonadism, which include a noticeable decrease in libido, erectile dysfunction, and persistent fatigue. Other signs can involve a reduction in lean muscle mass, a loss of body hair, and changes in mood, such as increased irritability or depression.
Low free testosterone in women can also result in non-specific symptoms like persistent fatigue and reduced sexual desire. However, women are more often tested for high free testosterone, which can indicate hyperandrogenism. High levels in women can lead to symptoms like hirsutism, which is the growth of excessive, coarse hair in a male-like pattern on the face or chest, along with acne and irregular or absent menstrual periods.
In men, high free testosterone is less common but can be related to the use of performance-enhancing substances or certain medical conditions. Excessive levels may contribute to increased aggression, severe acne, or polycythemia (an abnormally high concentration of red blood cells). A healthcare provider is necessary to interpret results accurately, as the “normal” range varies widely based on the measurement method, age, and sex.