Free Testosterone

What is free testosterone?

Most of the testosterone circulating in the blood is tightly bound to a protein called SHBG (sex hormone-binding globulin), and some is loosely bound to albumin. The small portion that is not bound to any protein is called free testosterone. Because this portion can pass directly into cells and exert the effects of a male hormone, it is considered the biologically active part of the hormone. The free portion makes up only a few percent of total testosterone.

Free testosterone can be measured directly with special methods, or it can be calculated from the total testosterone, SHBG and albumin values. Because there can be marked differences between methods, it is recommended that follow-up measurements be done at the same laboratory. When evaluated together with total testosterone, it gives a more accurate idea of how much hormone the body can actually use.

When is a free testosterone test ordered?

This test is usually ordered when total testosterone is borderline or when the SHBG level may have changed. Obesity, type 2 diabetes, older age, and thyroid and liver diseases can affect SHBG, so that total testosterone does not reflect the true situation. In women, it may be more sensitive than total testosterone in detecting androgen excess. In men who have symptoms of testosterone deficiency but a normal or borderline total value, free testosterone can make the decision easier.

• Reduced sex drive, erection problems or constant fatigue in men
• Borderline total testosterone
• Increased hair growth, acne or irregular periods in women
• Evaluation of polycystic ovary syndrome (PCOS)
• Diseases or medications that affect SHBG

How do I prepare for a free testosterone test?

Testosterone levels are highest in the morning and decrease during the day; for this reason, giving blood early in the morning is recommended. Because the value can drop somewhat after a meal, the measurement is usually done after an overnight fast. For female patients, the doctor may set the day of sampling according to the menstrual cycle. If a repeat measurement is needed, giving the sample again in the morning on an empty stomach allows the results to be compared.

Medications containing hormones, especially birth control pills, can affect free testosterone by changing the SHBG level; mention all the medications you take. If the measurement is done with an immunoassay, high-dose biotin (vitamin B7) can also affect the result. Avoid strenuous exercise the day before the test. If leaving home in the morning is difficult for you, a nurse can also take the blood sample at your home.

What does a free testosterone result mean?

In men, low free testosterone suggests that the amount of hormone reaching the tissues may be insufficient, even if the total value looks normal. Testicular problems, pituitary gland diseases, obesity and some medications are among the possible causes. In women, high free testosterone supports a hormonal cause for symptoms such as excess hair growth and acne; the most common cause of this picture is polycystic ovary syndrome. In men, a marked elevation points to the use of products containing testosterone.

No decision is made on the basis of a single low result; your doctor usually has the measurement repeated on another morning. Values vary considerably by age, sex, the method used and the laboratory; for this reason, the laboratory's reference information on this page is also given by age and sex. What your result means should be interpreted by your doctor together with total testosterone, SHBG, your symptoms and your other hormone tests.