Free testosterone (serum/plasma)

Free testosterone measures the small fraction of testosterone in your blood that is not bound to proteins such as sex hormone-binding globulin (SHBG) or albumin. Because this unbound portion is the biologically active form of the hormone, the test helps your doctor see how much usable testosterone is actually circulating in your body and can reveal hormonal imbalances that a total testosterone test alone might miss.

Free testosterone is the portion of total testosterone that is not bound to proteins (like sex hormone-binding globulin) and is therefore available to bind to receptors in your body. This form plays a key role in regulating male and female sexual development, muscle mass, bone density, and overall energy levels. Levels can vary based on age, sex, and health conditions.

Your doctor may order this test if you have symptoms that suggest abnormal testosterone levels, such as fatigue, low sex drive, infertility, erectile dysfunction, irregular menstrual periods, excessive body hair, or unexplained muscle loss. It is also used to monitor hormone therapy, evaluate conditions affecting the testes, ovaries, or adrenal glands, and to investigate early or delayed puberty in adolescents.

Your healthcare provider may order a free testosterone test if you have symptoms like low libido, fatigue, muscle weakness, or changes in hair growth. It may also be used in fertility evaluations, hormone replacement therapy planning, or to investigate conditions like hypogonadism (low testosterone) or Cushing’s syndrome.

This test requires a small blood sample, usually drawn from a vein in your arm by a trained phlebotomist. The sample is often collected in the morning, when testosterone levels are naturally at their highest. Your provider may ask you to fast for several hours beforehand and to avoid vigorous exercise, alcohol, or certain medications that can affect hormone levels before the sample is taken.

This test requires a blood sample drawn from a vein, typically in a clinic or lab setting. The sample is collected in a special tube that separates serum or plasma from blood cells. You may be asked to fast for 8–12 hours before the test, though this is not always required.

Before the test, tell your healthcare provider about all prescription drugs, over-the-counter medicines, vitamins, and herbal supplements you use, because some of them can change your testosterone readings. Wear a shirt with loose sleeves so your arm can be accessed easily, and let the staff know if you have ever felt faint during a blood draw, bruise easily, or take blood-thinning medications.

No special preparation is needed beyond standard blood draw precautions, such as avoiding strenuous activity or smoking for 30 minutes before the test. If you’re on medications (e.g., steroids), let your provider know, as they may affect results.