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What Is the Difference Between Free Testosterone and Bioavailable Testosterone?

by: Payam Hakimi
November 2024
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Testosterone, a vital hormone in both men and women, plays a critical role in maintaining muscle mass, bone density, energy levels, mood, and libido. When evaluating testosterone levels, doctors often measure free testosterone and bioavailable testosterone, two terms that provide insight into how much testosterone is accessible for the body’s use. While they are related, these terms represent different aspects of testosterone's activity in the body. Here's what sets them apart:

Free Testosterone

Free testosterone refers to the portion of testosterone in the bloodstream that is not bound to proteins like sex hormone-binding globulin (SHBG) or albumin. This unbound testosterone, typically accounting for 1–2% of total testosterone, is in its active form and readily available to interact with cells and receptors in the body. Because it is unbound, free testosterone can immediately exert its effects on tissues, making it a key indicator of hormonal activity.

Bioavailable Testosterone

Bioavailable testosterone encompasses all testosterone that is readily available for the body’s use. This includes free testosterone as well as testosterone loosely bound to albumin, a protein that binds less tightly than SHBG. While testosterone bound to SHBG is essentially inactive and unavailable to tissues, albumin-bound testosterone can easily dissociate and act as needed, increasing the pool of accessible testosterone. As a result, bioavailable testosterone represents a larger portion of active testosterone compared to free testosterone alone, typically 30–50% of total testosterone.

Key Differences

Binding Proteins:
- Free testosterone is completely unbound.
- Bioavailable testosterone includes free testosterone and testosterone loosely bound to albumin.
Percentage of Total Testosterone:
- Free testosterone makes up a very small fraction (1–2%).
- Bioavailable testosterone is a larger fraction (30–50%).
Clinical Relevance:
- Free testosterone provides a narrow snapshot of hormonal activity.
- Bioavailable testosterone offers a broader view of the testosterone that can influence bodily functions.

Why These Metrics Matter

Low levels of free or bioavailable testosterone can indicate hormonal imbalances or conditions like hypogonadism, even if total testosterone levels appear normal. Measuring both metrics can help doctors better understand a patient’s hormonal status and guide treatment decisions.

Both total testosterone and bioavailable testosterone typically increase with testosterone therapy, but the extent of the increase can depend on various factors such as the individual’s baseline levels, type of therapy, and response to treatment.

How Testosterone Therapy Affects Levels:

Total Testosterone:
Total testosterone includes all testosterone in the bloodstream—bound and unbound. Testosterone therapy generally leads to a significant increase in total testosterone levels, as it directly increases the overall pool of testosterone available in the body.
Bioavailable Testosterone:
Bioavailable testosterone, which includes free testosterone and testosterone loosely bound to albumin, also increases with testosterone therapy. This is because therapy raises the overall amount of testosterone, and more of it becomes available for biological activity.

Factors Influencing Bioavailable Testosterone Response:

SHBG Levels: Sex hormone-binding globulin (SHBG) binds tightly to testosterone, rendering it inactive. If SHBG levels are high, the proportion of bioavailable testosterone may increase more modestly compared to total testosterone. Conversely, individuals with low SHBG levels may experience a more significant rise in bioavailable testosterone relative to total testosterone.
Type of Therapy: Different forms of testosterone therapy (e.g., injections, creams, gels, patches) can have varying effects on how much testosterone becomes free or bioavailable.

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What Is the Difference Between Free Testosterone and Bioavailable Testosterone?

Free Testosterone

Bioavailable Testosterone

Key Differences

Why These Metrics Matter

How Testosterone Therapy Affects Levels:

Factors Influencing Bioavailable Testosterone Response:

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Take care of yourself and your loved ones this holiday season with our holiday immunity pack- Includes Vitamin D3, C, Methylated B Complex, and More!https://bodyofharmony.com/a/bundles/holiday-immunity-byjl

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Subtotal: $0.00

What Is the Difference Between Free Testosterone and Bioavailable Testosterone?

Free Testosterone

Bioavailable Testosterone

Key Differences

Why These Metrics Matter

How Testosterone Therapy Affects Levels:

Factors Influencing Bioavailable Testosterone Response:

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Share This Post

Prev Post

"ME/CFS: A Complex, Long-Term Illness Affecting Multiple Body Systems"

Next Post

Who Needs Testosterone Replacement Therapy?

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Search

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Why Daily Testosterone Creams Are Physiologically Superior to Weekly Injectable Testosterone

Understanding High hs-CRP and Natural Ways to Lower It

Understanding Prolactin: A Hormone for Both Men and Women

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Recently Bought