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Free testosterone is the portion of testosterone in your body that is readily available for use. Also known as bioavailable testosterone, it’s the form that your cells can easily absorb and utilize. Most testosterone in your bloodstream is bound to proteins like SHBG (sex hormone-binding globulin), making it inactive. SHBG’s primary function is to transport and regulate sex hormones, including testosterone, controlling how much is accessible to your body at any given time. Typically, only 2% to 5% of your total testosterone is free and usable. When free testosterone levels are low, it can lead to symptoms of testosterone deficiency, affecting energy, mood, and overall well-being.
| Labs | Men |
|---|---|
| Total testosterone (ng/dL) | 250 - 1100 |
| Free testosterone (ng/dL) | 5 - 30 |
| Albumin (g/dL) | 3.5 - 5.5 |
Yes, it’s possible to have normal total testosterone levels but low free testosterone, which may lead to symptoms of testosterone deficiency.
As we’ve seen, it’s the bioavailable portion of testosterone that exerts its effects on the body. So, if SHBG levels are particularly raised relative to total testosterone, it won’t leave much free testosterone to act on tissues. This can be seen in older age, where SHBG levels often increase considerably.
Other causes of raised SHBG levels include:
First, let’s recap testosterone’s overall function in the body.
Testosterone is a primary sex hormone primarily made in the testes, but also in the ovaries and adrenal glands. It’s important for growth and development, fertility, and maintaining muscle and bone mass. Low levels can affect mood, energy levels, and sexual function.
The effects of testosterone are more clearly seen in men. Abnormal levels, low or high, can give rise to symptoms and may be due to an underlying condition.
Once testosterone is made, it’s released into the bloodstream. But because it doesn’t dissolve in water (or blood), most testosterone (about 95%) catches a ride on other proteins, like sex hormone-binding globulin (SHBG) and albumin.
All the time testosterone is bound to proteins, it’s rendered inactive. It can’t act on tissues until it’s released. There are two benefits to this:
Once it reaches its target cell, testosterone binds to specific receptors and triggers changes within the cell.
As we’ve seen, carrier proteins bind to and release testosterone within the bloodstream.
Total testosterone is a total measure of testosterone in the bloodstream — both free and bound. Levels are measured by an analyser directly.
Free testosterone is the concentration of testosterone in the blood that is not attached to proteins. Only about 2–5% of testosterone is free.
Measuring free testosterone directly is possible, but it’s challenging and not widely available. So it’s often calculated instead, using total testosterone, SHBG, and albumin levels. It’s a complicated equation that produces a very reliable estimate. Thankfully, the lab works this out for you.
Bioavailable testosterone reflects testosterone that is readily available and able to act to tissues. It makes sense that free testosterone falls into this category, since it can bind to cell receptors as it pleases.
But testosterone bound to albumin is also classed as bioavailable. That’s because the testosterone-albumin bond is easily reversible, meaning testosterone frequently switches between active and inactive forms. This effect is greatest in men as a greater proportion of their testosterone binds to albumin.
The remaining testosterone is bound to SHBG and this bond is more secure. This means that high levels of SHBG can reduce levels of free and bioavailable testosterone.
Yes, it’s possible to have normal total testosterone levels but low free testosterone, which may lead to symptoms of testosterone deficiency.
As we’ve seen, it’s the bioavailable portion of testosterone that exerts its effects on the body. So, if SHBG levels are particularly raised relative to total testosterone, it won’t leave much free testosterone to act on tissues. This can be seen in older age, where SHBG levels often increase considerably.
Other causes of raised SHBG levels include:
The opposite is also true: total testosterone levels may appear low when free testosterone levels are within normal range. This is most often seen in men in the development of insulin resistance (obesity or diabetes), which often causes SHBG levels to decline. Therefore, less testosterone is bound to proteins [1]. You can check your blood sugar with a Diabetes (HbA1c) Blood Test.
Free testosterone levels are dependent on both total testosterone levels and your levels of SHBG (the protein that binds to it).
Therefore, increasing your total testosterone level through healthy lifestyle choices is also likely to increase your free testosterone levels. Factors like stress, low levels of activity, and being overweight can all impact your testosterone production. There are many ways you can boost testosterone levels naturally, from losing weight to maintaining a healthy, balanced diet.
SHBG naturally increases with age, but you may be able to reduce this effect somewhat by making sure your diet contains plenty of protein [9]. If your levels are abnormally high, it’s worth exploring whether medications or an underlying health condition may be playing a part in this.
You can also schedule a consultation with our providers to find out more about how your body is performing as a whole, including free testosterone.
Mantality has been helping men live better for more than 10 years. We are the leader in testosterone replacement therapy throughout the Midwest since 2013.
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