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Free testosterone is the active unbound version of the hormone. Levels in the blood depend on total testosterone production, clearance, and how much is bound to proteins in the blood. Sex hormone binding globulin (SHBG) is the major binding protein and as it increases, free testosterone decreases.
It is important to have enough free testosterone to carry out major functions such as muscle maintenance, red blood cell production, and support of cardiometabolic health.
Low free testosterone can be associated with late onset hypogonadism and worsening of associated symptoms, anemia, metabolic syndrome, diabetes, high blood pressure, reduced muscle mass, and chronic opioid use. In those diagnosed with prostate cancer, a lower free testosterone was associated with high-grade versus low or intermediate grade disease,
It is important to assess free testosterone when SHBG levels are altered or when total testosterone is declining.
The percentage of testosterone that is free (%FT) can be determined by dividing free testosterone by total testosterone. Usually just a small percentage of testosterone in circulation is free and unbound, usually just 1-3% of total. However, the percentage can be altered by changes in binding proteins, especially sex hormone binding globulin, as well as other factors including age, obesity, and alcohol consumption.
Although the relationship between testosterone and prostate cancer is complex, research suggests the the %FT may be higher in men with more aggressive high-grade prostate cancer compared to those with low-intermediate grade disease.
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