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Decoding Testosterone: Bound, Free, and Bioavailable Explained

Updated: Sep 14, 2023

There are three types of testosterone. In day-to-day conversations about testosterone, the numbers for total testosterone are usually used. Although usually adequate, total testosterone is the least important. Free and Bioavailable testosterone levels are far more important because they represent the usable testosterone in the body.

Testosterone is essential for men's health
Total, Free and Bioavailable Testosterone

Many doctors will only test total testosterone, but if you get your testosterone tested, you should have all three tested, along with SHBG and Estradiol. Insist on it. Also, don't let your doctor get away with telling you your levels are "normal," so everything is fine. Normal is not the same as healthy or optimal testosterone levels. I will discuss this in future blog posts.

Bound Testosterone: This is testosterone that is bound to proteins in the blood. The majority of testosterone in the body is bound testosterone. It's primarily attached to a protein called sex hormone-binding globulin (SHBG); a smaller amount is bound to albumin.

Free Testosterone: This testosterone is not bound to any proteins and freely circulates in the bloodstream. This form of testosterone is available to the cells and can directly affect the body. This is the active form of Testosterone and is the most important regarding your health.

Bioavailable Testosterone: This is the testosterone bound to albumin. Testosterone bound to albumin can be easily converted into free testosterone, making it bioavailable.

Total Testosterone: is the sum of Bound, Free, and Bioavailable Testosterone. This number is usually the number used to determine if you have healthy levels. Usually, it is good enough, but Free Testosterone is a much better indicator.

Free and bioavailable testosterone are usually measured using a series of calculations rather than direct measurements, as direct measurements can be challenging and costly.

Free Testosterone

The calculation for free testosterone typically involves knowing the levels of total testosterone and SHBG. Albumin is usually considered a constant (4.3 g/dL) because it varies little in individuals.

One commonly used formula for calculating free testosterone is:

Free testosterone (ng/dL) = Total testosterone x (1 - SHBG / (SHBG + albumin))

Bioavailable Testosterone

Bioavailable testosterone is calculated using the total testosterone, SHBG, and albumin levels, similar to the free testosterone calculation but with some differences.

The Vermeulen formula is commonly used:

Bioavailable Testosterone (nmol/L) = Total Testosterone - [SHBG * (Total Testosterone / (Kd + 1))]

In the Vermeulen formula, Kd is the equilibrium dissociation constant of testosterone from SHBG, usually considered as 1x10^-9 M.

These formulas, while useful, have their limitations and may not be entirely accurate due to the complexity of hormone interactions in the body. Therefore, they're best used as part of a broader assessment of hormonal health, not in isolation.

Free and bioavailable testosterone can be directly tested through direct assays. These tests are often used when there is a suspicion that the body's sex hormone-binding globulin levels are abnormal, which could alter the amount of free or bioavailable testosterone.

However, these direct tests are not always accurate or reliable, so some physicians prefer to calculate free or bioavailable testosterone levels. That's why the method used can vary from one healthcare provider to another. It can also depend on the specific circumstances of the patient.

It's important to note that normal levels can vary widely among individuals, and what is considered "normal" can also vary depending on the laboratory that analyzes the blood samples. Again, It is also important to understand that normal levels are not the same as healthy or optimal testosterone levels.

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