Testosterone is a hormone that plays a role in the health of both men and women, affecting everything from bone density to mood regulation. While the total amount of the hormone in the bloodstream is often measured, this number does not reveal the full picture of the hormone’s activity. The truly active component that interacts with the body’s cells is free testosterone, and understanding its level is necessary for a health assessment. This specific measurement is often a more reliable indicator of potential hormonal imbalance than the total concentration.
Defining Free Testosterone
The body produces testosterone, but the vast majority of it is not immediately available for use by tissues. Testosterone circulates in the blood in two forms: bound and unbound. The bound form is attached to carrier proteins, primarily Sex Hormone Binding Globulin (SHBG) and, to a lesser extent, albumin.
The portion of testosterone that is not bound to these proteins is called free testosterone. This unbound fraction is biologically active because it can easily leave the bloodstream and enter cells throughout the body to engage with androgen receptors. Free testosterone is the fraction responsible for the hormone’s effects on muscle mass, libido, and energy levels.
Establishing Reference Ranges
Normal free testosterone levels vary significantly based on biological sex and age, and the ranges can also differ slightly between laboratories due to the specific testing method used. The measurement is typically reported in picograms per milliliter (pg/mL) or nanograms per deciliter (ng/dL). The appropriate range must be interpreted within the context of the patient’s demographics and symptoms.
Males
For adult men, the free testosterone level naturally declines with age, making age-specific reference ranges particularly important. In young adult men, typically those aged 19 to 39 years, the range measured by standardized methods is approximately 120 to 368 pg/mL. For the broader population of adult men (19 years and older), a range of 66 to 309 pg/mL is often cited as a general reference.
Females
Women produce much lower levels of testosterone, and their reference ranges are substantially lower than those for men. For pre-menopausal women, the normal free testosterone range often falls between 0.3 and 1.9 pg/mL. Post-menopausal women typically have lower levels, with ranges sometimes cited between 0.3 and 0.65 pg/mL.
Clinical Significance of Abnormal Levels
Levels of free testosterone that fall outside the established reference ranges can lead to a variety of physical and psychological symptoms, with implications differing greatly between men and women.
Low Free Testosterone
In adult men, an abnormally low free testosterone level is often associated with symptoms of hypogonadism. These manifestations include a decrease in libido and sexual desire, along with erectile dysfunction. Physical changes can involve a loss of muscle mass and strength, an increase in body fat, and a reduction in bone density. Men may also experience psychological effects such as fatigue, depressed mood, and difficulty with concentration and memory. For women, low free testosterone levels can result in a reduced sex drive, persistent fatigue, muscle weakness, and mood changes.
High Free Testosterone
In men, excessively high free testosterone levels, often resulting from hormone supplementation, can lead to adverse effects despite the hormone’s anabolic nature. Symptoms may include an increased risk of prostate enlargement, a higher hematocrit percentage, and an elevated risk of cardiovascular issues. Paradoxically, very high levels can suppress the body’s natural production, potentially leading to low sperm count and infertility. In women, high free testosterone is a hallmark of hyperandrogenism, often associated with Polycystic Ovary Syndrome (PCOS). Symptoms include hirsutism, acne, irregular or absent menstrual cycles, and, in more severe cases, a deepening of the voice.
Interpretation and Measurement
Accurate measurement of free testosterone is a complex process, and the testing method significantly influences the result. The gold standard for measuring the free fraction is equilibrium dialysis, a labor-intensive technique that separates the unbound hormone from the bound proteins. Due to its complexity, many laboratories rely on calculated free testosterone, which uses a formula based on measured total testosterone and SHBG levels. The time of the blood draw is also a critical factor because testosterone levels follow a circadian rhythm, peaking in the early morning. Samples should ideally be collected between 8:00 a.m. and 10:00 a.m.