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Testosterone Free and Weakly Bound Blood Test

Free and weakly bound testosterone (FWBT), also referred to as bioavailable testosterone, is thought to reflect an individual's biologically active, circulating testosterone. FWBT includes free testosterone and testosterone that is bound to albumin. FWBT does not include sex hormone binding globulin-bound testosterone.

Sample Report

Test Code: 143255

CPT Code: 84410

Also Known As: Bioavailable Testosterone, Free and Albumin-Bound Testosterone, FWBT


Ammonium sulfate precipitation; radioassay

Specimen Type: Blood


Stop biotin consumption at least 72 hours prior to the collection.

Test Results:


Free and weakly bound (bioavailable) testosterone measurement involves the selective precipitation of SHBG with ammonium sulfate. Tritiated testosterone is added to serum which is then allowed to come to equilibrium at physiologic temperature. Testosterone bound to SHBG is then selectively precipitated with 50% ammonium sulfate, leaving free and albumin-bound testosterone in solution. The percentage of tritiated label not bound to SHBG is multiplied by the total testosterone to produce the bioavailable testosterone.

Elevated levels of FWBT are observed in female hirsutism.2 The measurement of free and weakly bound testosterone in women, when used in conjunction with the assay of the DHEA-S and SHBG, can be used to establish etiology of hirsutism. In males, decreased serum concentrations are associated with hypogonadism. FWBT levels tend to increase during pregnancy but have been found to remain below the upper limit of the reference interval.3 Total testosterone levels in women decrease by approximately 30% after menopause.4 Administration of exogenous estrogens has the physiologic effect of increasing SHBG concentrations and suppressing the production of androgens by the ovary.4 This results in a net decrease in FWBT. Decreased FWBT levels have been associated with diminished libido4 and loss of bone density.5 FWBT levels in males fall with age6 at a rate that exceeds that of total testosterone and parallels the drop in DHEA sulfate. This decrease is thought to be caused by diminished testicular production and not due to hypothalamic/pituitary insufficiency.7 Decreased FWBT was not, however, found to correlate with diminished potency.8 Since SHBG has been found to increase with age, the FWBT level may be a more reliable indicator of testosterone production than total testosterone.

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