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Free Testosterone Blood Test: Measuring the Active Form of Testosterone

The Free Testosterone Blood Test, part of the PlexusDx Men’s Hormone Blood Test, measures the small fraction of testosterone (about 2–3%) that is not bound to proteins such as sex hormone-binding globulin (SHBG) or albumin. This “free” testosterone is the biologically active form, directly available to tissues and responsible for many of testosterone’s critical functions in men’s health.

Why Free Testosterone Matters

Total testosterone testing measures all circulating testosterone, but it doesn’t show how much is actually usable. Since most testosterone is protein-bound and inactive, free testosterone provides a more accurate reflection of hormone activity. This is especially important when symptoms of low testosterone appear despite total levels falling within the “normal” range.

Functions of Free Testosterone in Men

Testosterone is primarily produced by the testes in response to luteinizing hormone (LH) released from the pituitary gland. Free testosterone, in particular, plays a direct role in many key aspects of men’s health, including:

  • Bone density: Helps maintain strong bones and reduce fracture risk.
  • Muscle mass: Supports muscle growth, strength, and performance.
  • Red blood cell production: Stimulates bone marrow to produce red blood cells for oxygen delivery.
  • Sexual health: Essential for libido, erectile function, and sperm production.
  • Mood and brain health: Influences memory, focus, and emotional well-being.

How Free Testosterone Levels Change with Age

Free testosterone levels typically peak in a man’s late 20s and then gradually decline with age. This decline can contribute to andropause (sometimes referred to as “male menopause”), which may bring symptoms such as:

  • Fatigue and low energy
  • Reduced libido and sexual function
  • Loss of muscle mass and strength
  • Increased body fat
  • Mood changes such as depression or irritability
  • Cognitive changes, including reduced focus or memory

Why Test for Free Testosterone

The Free Testosterone Blood Test is particularly useful when:

  • Symptoms don’t match total testosterone results: A man may have “normal” total testosterone but low free testosterone, explaining symptoms of deficiency.
  • SHBG levels are abnormal: Since SHBG binds testosterone, changes in SHBG can alter free testosterone even if total levels remain steady.
  • Evaluating fertility: Free testosterone plays a role in sperm production and reproductive health.
  • Assessing metabolic and cardiovascular risk: Research shows that low free testosterone is associated with higher cardiovascular risk and metabolic disorders.

Reference and Functional Ranges

Lab reference ranges vary, but typical values for adult men include:

  • Standard reference range: 5 – 21 ng/dL (varies by lab)
  • Functional/optimal range: 10 – 20 ng/dL, depending on age and health goals

For women, free testosterone levels are much lower, typically less than 1 ng/dL.

Factors That Influence Free Testosterone

Several factors can influence free testosterone levels:

  • Age: Natural decline begins after the late 20s.
  • SHBG levels: High SHBG reduces free testosterone, while low SHBG increases it.
  • Body composition: Obesity and high body fat lower free testosterone.
  • Lifestyle: Stress, poor sleep, inactivity, and excessive alcohol can all reduce free testosterone.
  • Health conditions: Diabetes, metabolic syndrome, thyroid disorders, and liver disease can alter hormone levels.

How the Test Works

The PlexusDx Free Testosterone Blood Test is performed using an at-home dried blood spot collection kit (ADX card). This method allows you to collect a sample easily and conveniently, without the need for a traditional blood draw. Once mailed to the lab, your results are analyzed and delivered securely online.

Optimizing Free Testosterone Levels

When free testosterone levels are low, several lifestyle and medical strategies can help:

  • Exercise: Resistance and strength training increase testosterone naturally.
  • Weight management: Reducing body fat improves testosterone availability.
  • Sleep: Restorative sleep is critical for testosterone production.
  • Stress reduction: Lowering cortisol through relaxation techniques supports hormone balance.
  • Medical options: In cases of true hypogonadism, doctors may recommend testosterone replacement therapy (TRT).

Key Takeaways

  • The Free Testosterone Blood Test measures the active form of testosterone that directly impacts health.
  • Free testosterone is essential for bone strength, muscle mass, libido, fertility, mood, and brain health.
  • Levels peak in the late 20s and gradually decline with age, contributing to andropause symptoms.
  • Low free testosterone is linked not only to fatigue and sexual health concerns but also to increased cardiovascular risk.
  • PlexusDx makes testing simple with at-home dried blood spot collection and fast, reliable results.

Final Thoughts

The Free Testosterone Blood Test is one of the most valuable tools for understanding men’s hormone health. By focusing on the active fraction of testosterone, it provides clear insight into symptoms and risks that may be missed when looking only at total testosterone. With the convenience of at-home testing, PlexusDx empowers you to monitor, understand, and take action on your hormone health for improved energy, vitality, and long-term well-being.