If you had your testosterone (T) tested, then you should consider calculating your free and bioavailable T levels.
T in both men and women is mostly bound to proteins inside the blood, such as albumin and sex hormone-binding globulin (SHBG). This renders the hormone either temporarily or completely inactive. Measuring total testosterone alone might be insufficient for a proper diagnosis which is why calculating free and bioavailable testosterone is recommended.
Remember that your blood tests or calculator results are not a diagnosis
To confirm any hormonal problems with low or high T, you should talk to an experienced doctor who will evaluate your health status, symptoms, and medical history.
What is Free Testosterone?
Normally, more than 98% of the testosterone in your blood is bound to either SHBG (50-60%) or albumin (40-50%). This prevents it from producing its effects directly to the tissues. Free testosterone (FT) is the form of T in your blood that is not bound by any proteins. This is the active form of testosterone which produces the typical T effects including regulating sexual function and body composition.
Free testosterone is considered more essential in the diagnosis of hypogonadism than total testosterone
Usually, free T levels depend on the amount of total T. However, certain conditions can affect the levels of SHBG and change the ratio between free T and bound T without affecting total T. In such cases, free testosterone is considered more important in the diagnosis of hypogonadism than total T.
One of these conditions is aging. According to evidence, it can lead to low FT coupled with symptoms whilst the person still has normal total T levels. For example, a study in 3334 men showed that males with low free T but normal total T had more symptoms of hypogonadism compared to those with low total T but normal free T levels.
SHBG can also surge during extreme weight loss, liver disease, and severe chronic infections. All these conditions diminish FT. In women, pregnancy also increases SHBG.
High free T in men is rare. It is either due to hormone-producing tumors or the use of anabolic steroids. In comparison, women have higher SHBG levels than men, which keeps most of the testosterone inactivated.
Besides, conditions such as obesity, diabetes, and polycystic ovarian syndrome (PCOS) often manifest with serious insulin resistance. This involves high insulin levels and systemic inflammation which are associated with low SHBG and consequently high free T in women.
What are the Average Free Testosterone Levels by Age?
Normally, FT should be around 2% of your total T. Optimal free testosterone levels in men are between 5-12 ng/dL (0.4-0.2 nmol/L) depending on age. Normal free testosterone levels in women are between 0.06-1.00 ng/dL, again depending on age.
Free testosterone levels can be measured by dialysis or immunoassay. Equilibrium dialysis is the most precise method, but it’s expensive and laborious which makes it impractical. On the other hand, the immunoassay is affordable but imperfect.
Below is a chart of the normal testosterone ranges according to age and gender (references were estimated using morning measurements with are 50% higher on average than afternoon levels):
What is Bioavailable Testosterone?
Bioavailable testosterone (BT) accounts for all the T in your blood that is not bound to SHBG. When SHBG binds with testosterone, your tissues can no longer use it – i.e. it is not bioavailable. The bioavailable fraction consists of your free T and albumin-bound testosterone.
Compared to FT, albumin-bound testosterone is not bioactive. However, unlike SHBG, albumin binds weakly to testosterone. Under certain conditions, the hormone can dissociate from this complex which would release free testosterone inside the tissues. Because of this, both albumin-bound testosterone and free T are considered bioavailable.
What is the Normal Range for Bioavailable Testosterone?
Normal bioavailable testosterone in men ranges between 40-260 ng/dL. Normal bioavailable testosterone in women is 0.8-10 ng/dL.
According to studies, the BT levels are an important indicator that correlates strongly with age and symptoms of low T.
In fact, measuring bioavailable testosterone levels via immunoassay reflects much more precisely the total bioactive T than measuring free T.
Both free T and total bioavailable T can be accurately calculated using total T, SHBG, and albumin concentrations
How to Increase Free and Bioavailable Testosterone Levels?
Nutritional deficiencies can have an impact on total T levels, as well as the ratio between SHBG-bound and BT.
For example, the essential mineral magnesium reduces the affinity of SHBG to T and thus increases bioavailable T levels. Studies have shown that magnesium supplementation can increase both total and free T. If you want to get enough magnesium naturally you should eat more pumpkin seeds, avocadoes, nuts, beans, and spinach.
Another nutrient that reduces SHBG and peaks FT is boron. A trial in 8 healthy men showed that free testosterone increased by 28% after a week of supplementation. The most common natural sources of boron are avocado, peanuts, apples and dried beans.
Vitamin D is another commonly deficient nutrient. A study in 200 participants deficient in vitamin D revealed that supplementation for a year increased free T by 20%. Your body can synthesize enough vitamin D only if you have adequate sun exposure. Additionally, foods such as salmon, sardines, and mackerel are the best sources.
Unfortunately, low T levels are sometimes caused by a testosterone deficiency. This is a medical condition called hypogonadism where the body does not produce enough T due to various medical reasons.
If it is coupled with symptoms of low T, then your doctor might recommend testosterone replacement therapy (TRT). The therapy is proven as an effective tool in increasing serum testosterone levels, boosting FT and lowering SHBG.
Measuring only your total T levels can be insufficient for determining whether or not you have a testosterone deficiency because a large percentage of the hormone is bound to proteins in your blood such as albumin and SHBG.
Estimating your free and bioavailable T can allow for accurate diagnosis and adequate treatment of testosterone deficiency
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