From the Guidelines
The main difference between total and free testosterone is that total testosterone measures all testosterone in the blood, while free testosterone measures only the unbound, biologically active portion. Total testosterone includes both bound testosterone (attached to proteins like sex hormone-binding globulin and albumin) and free testosterone. Typically, only about 2-3% of total testosterone exists in the free form, but this small percentage is responsible for most of testosterone's effects in the body, as noted in a study published in 2024 1. Free testosterone is the portion that can enter cells and bind to receptors to produce masculinizing effects and influence metabolism, muscle growth, and sexual function. When testing hormone levels, doctors often measure total testosterone first, but free testosterone provides more useful information about hormone activity, especially in cases where binding proteins may be abnormal, such as in obesity, as discussed in a 2017 study 1. Conditions like obesity, liver disease, or thyroid disorders can affect binding proteins, creating situations where total testosterone appears normal while free testosterone is low, or vice versa. For comprehensive hormone assessment, measuring both forms provides the most complete picture of testosterone status. Some key points to consider when evaluating testosterone levels include:
- Measuring morning total testosterone levels using an accurate and reliable assay 1
- Determining free testosterone concentrations either directly from equilibrium dialysis assays or by calculations that use total testosterone, sex hormone binding globulin, and albumin concentrations, especially in men with total testosterone levels close to the lower limit 1
- Considering the potential effects of obesity on testosterone levels, including decreased sex hormone-binding globulin and increased aromatization of testosterone to estradiol in adipose tissue 1
- Evaluating patients with signs and symptoms of hypogonadism, including decreases in energy, libido, muscle mass, and body hair, as well as hot flashes, gynecomastia, and infertility 1
From the FDA Drug Label
Circulating testosterone is primarily bound in the serum to sex hormone-binding globulin (SHBG) and albumin. Approximately 40% of testosterone in plasma is bound to SHBG, 2% remains unbound (free) and the rest is loosely bound to albumin and other proteins Testosterone in plasma is 98 percent bound to a specific testosterone-estradiol binding globulin, and about 2 percent is free.
The difference between total and free testosterone is the way testosterone is bound in the blood.
- Total testosterone refers to the total amount of testosterone in the blood, including both bound and free testosterone.
- Free testosterone refers to the amount of testosterone that is not bound to any proteins in the blood, which is approximately 2% of the total testosterone. The majority of testosterone (about 98-40%) is bound to proteins such as sex hormone-binding globulin (SHBG) and albumin, and is not available for use by the body. 2 3
From the Research
Definition and Measurement of Total and Free Testosterone
- Total testosterone refers to the total amount of testosterone present in the blood, including both bound and free testosterone [ 4 ].
- Free testosterone, on the other hand, refers to the amount of testosterone that is not bound to proteins in the blood and is available to enter cells and exert its effects [ 4 ].
Differences in Measurement and Clinical Significance
- The measurement of total and free testosterone levels is essential for the diagnosis of androgen disorders [ 4 ].
- Total testosterone should be measured in a fasting state in the morning using a reliable assay, while free testosterone levels should be measured in men with suspected alterations in binding protein concentrations or in whom total testosterone levels are only slightly above or below the normal range [ 4 ].
Clinical Applications and Treatment
- Testosterone replacement therapy (TRT) aims to improve symptoms and signs of testosterone deficiency by increasing serum testosterone levels to the physiologic range [ 5 ].
- Different routes of delivery and formulations of TRT have been developed to improve pharmacokinetics and patient compliance [ 5 ].
- The choice of TRT modality may affect total testosterone levels, as well as other parameters such as estradiol and hematocrit [ 6 ].