Testosterone is an important hormone for both men and women. It plays a key role in growth, development, fertility, and keeping your muscles and bones strong.
When you have your testosterone levels tested, it’s important to understand what those levels mean. Not all the testosterone in your blood is the same. Some of it is “total” testosterone, while some is “free” testosterone, and some is “bioavailable.”
In this article, we’ll take a close look at what’s known as the free testosterone index and why it matters. We’ll also review how to get tested and what the tests mean.
Understanding Testosterone: Total vs. Free vs. Bioavailable
Testosterone, an androgen hormone, is important for both men and women.
- In men, the testes produce it and it contributes to male characteristics.
- In women, the adrenal glands and ovaries produce it in smaller amounts.
What are normal testosterone levels? The Endocrine Society defines normal as:
- Males: 291 to 1,100 nanograms per deciliter (ng/dL) total testosterone
- Females: 18 to 54 ng/dL total testosterone
Most testosterone travels through your bloodstream attached to proteins. About 95% of it is bound to proteins like sex hormone-binding globulin (SHBG) and albumin.
SHBG holds onto testosterone tightly, while albumin’s grip is a bit looser.
So, what’s the difference between “total” and “free” testosterone?
“Total testosterone” is the measurement of all the testosterone in your blood, whether it’s attached to a protein or not.
“Free testosterone” is the testosterone that’s unattached, and only a small amount of your testosterone is “free” — about 2-5%. This is the only kind of testosterone that’s biologically active.
“Bioavailable testosterone” is all the free testosterone plus the testosterone that’s attached to albumin. Because albumin doesn’t hold onto testosterone very tightly, this kind of testosterone is easily available for your body to use.
Calculating the Free Testosterone Index (FAI)
The Free Androgen Index (FAI) is a quick way to estimate your free testosterone levels by using a simple formula that relies on your total testosterone and SHBG (sex hormone-binding globulin) levels. It’s particularly useful when directly measuring free testosterone is difficult or too expensive.
The formula is: (Total Testosterone / SHBG) x Constant.
The “constant” depends on the units used for testosterone and SHBG, so it’s important to use the correct one. This calculation provides an estimate of your free testosterone, so it’s not quite as precise as direct measurement, but it’s a good starting point.
Why Test Free Testosterone?
Measuring free testosterone levels can help to identify hormone imbalances that might not be clear from a total testosterone test alone.
Symptoms of Low and High Testosterone
Low testosterone, also known as hypogonadism, can cause a range of symptoms in men, including:
- Loss of sex drive
- Low energy
- Bone loss
- Infertility
High testosterone in women can manifest in the following ways:
- Infertility
- Irregular periods
- Acne
- Obesity
- Hirsutism (excessive hair growth)
When is Free Testosterone Testing Recommended?
Free testosterone testing is often recommended when doctors suspect that a person’s testosterone levels are out of balance, especially if their total testosterone levels are normal or borderline.
This test can be particularly helpful in evaluating conditions like polycystic ovary syndrome (PCOS) in women and hypogonadism in men.
It’s also useful when sex hormone-binding globulin (SHBG) levels are suspected to be abnormal, as SHBG affects the amount of testosterone that’s available for your body to use.
Factors Affecting Free Testosterone Levels
Many different things can affect your testosterone levels. These include:
Medical Conditions
Certain health conditions, such as polycystic ovary syndrome (PCOS), Cushing’s syndrome, and thyroid disorders, can throw off your hormone levels.
Medications and Substances
Did you know that some medicines, herbs, vitamins, supplements, like alpha-lipoic acid, and even recreational drugs can affect your androgen levels? That’s why it’s so important to tell your doctor about everything you’re taking, even if it’s something you bought over the counter.
Lifestyle Factors
Your lifestyle choices can also play a role. For example, drinking too much alcohol, being overweight, and having diabetes can all impact your testosterone levels.
Age
It’s also worth noting that testosterone levels naturally decrease as men get older. This is a normal part of aging, but it can still have an impact on your health and well-being.
Interpreting Your Free Testosterone Test Results
It’s crucial to have a healthcare provider interpret your free testosterone test results. Normal ranges vary significantly based on age, sex, and overall health, so don’t try to self-diagnose!
Sometimes, your free testosterone can be low even if your total testosterone levels appear normal. This can happen if you have high levels of SHBG, which binds to testosterone and makes it unavailable for your body to use. Certain medications, such as lisinopril, and medical conditions can cause elevated SHBG.
Your doctor may also order other tests (like LH, FSH, and prolactin) to figure out what’s causing abnormal free testosterone levels. All the results together will paint a clearer picture.
Testing considerations for specific populations
When should doctors order a free androgen index versus other hormone tests?
Men
For men, a total testosterone test is often the first test that doctors order.
Women
If a woman is experiencing symptoms of hyperandrogenism, which is when there are high levels of androgens in the body, doctors may order a total testosterone test, an SHBG test, and calculate the FAI.
Transgender and non-binary individuals
For transgender and non-binary people, a total testosterone test is usually the standard. However, if the total testosterone levels don’t match the person’s stage of transitioning, a doctor may order a free or bioavailable testosterone test.
Frequently Asked Questions
Do you want high or low free testosterone?
Generally, a healthy, normal range of free testosterone is what you’re aiming for. Too low, and you might experience symptoms like fatigue and decreased libido. Too high, especially in women, and it could lead to issues like acne or irregular periods. It’s all about balance, and that ideal level varies from person to person.
What does it mean if your free androgen index is high?
A high free androgen index (FAI), which is closely related to free testosterone, often suggests elevated levels of androgens, like testosterone. In women, this could indicate conditions like polycystic ovary syndrome (PCOS). In men, it might point to other hormonal imbalances. It’s best to discuss this with a healthcare provider to understand the underlying cause.
What is a normal free testosterone index?
Normal ranges for the free testosterone index vary depending on the lab and the individual. However, generally, for adult men, a normal range might be something like 9-30.3 ng/dL. For women, it’s much lower, often around 0.3-1.9 ng/dL. Always refer to the specific reference range provided by the lab that conducted your test.
Does low testosterone cause weight gain?
Low testosterone can contribute to weight gain, particularly a loss of muscle mass and an increase in body fat. Because muscle burns more calories than fat, a decrease in muscle mass can slow down your metabolism, making it easier to gain weight. It’s not the only factor, but it can definitely play a role.
What is the healthiest testosterone level?
There isn’t a single “healthiest” testosterone level that applies to everyone. It’s more about being within the normal range for your age and sex and, more importantly, not experiencing significant symptoms related to either high or low levels. It’s a conversation to have with your doctor, considering your overall health and well-being.
In conclusion
Understanding your free testosterone and free androgen index can help you understand your body’s androgen levels. But interpreting these tests can be tricky, especially when considering TRT coverage through programs like Medi-Cal. It’s important to talk with a healthcare professional to get the right interpretation and the right management plan for you.