Testosterone, Free and Weakly Bound
| Testosterone, Free and Weakly Bound | | | |
| Number | | 143255 |
| CPT | | 84402; 84403 |
| Related Information | | Free Androgen Index (FAI), Serum Testosterone, Free (Direct), Serum |
| Synonyms | | Bioavailable Testosterone ; Free and Albumin-Bound Testosterone |
| Test Includes | | Total testosterone |
| Specimen | | Serum |
| Volume | | 1.2 mL |
| Minimum Volume | | 0.6 mL (Note: This volume does not allow for repeat testing.) |
| Container | | Red-top tube or gel-barrier tube |
| Collection | | If a red-top tube is used, transfer separated serum to a plastic transport tube. |
| Storage Instructions | | Refrigerate at 2°C to 8°C. |
| Causes for Rejection | | Gross lipemia; recently administered isotopes |
| Reference Interval | | - % free and weakly bound: male: 9.0% to 46.0%, female: 3.0% to 18.0%
- Free and weakly bound: male: 40.0-250.0 ng/dL, female: 0.0-9.5 ng/dL
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| Use | | 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. The SHBG-bound fraction is biologically inactive because of the high binding affinity of SHBG for testosterone. The rapid dissociation of “weakly bound” testosterone from albumin results in the availability of essentially all albumin-bound testosterone for steroid-receptor interaction.1 |
| Methodology | | Ammonium sulfate precipitation; radioassay |
| Additional Information | | 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. |
| Footnotes | | - Pardridge WM, “Transport of Protein-Bound Hormones Into Tissues in vivo,” Endocr Rev, 1981, 2(1):103-23 (review).
- Cumming DC and Wall SR, “Nonsex Hormone-Binding Globulin-Bound Testosterone as a Marker for Hyperandrogenism,” J Clin Metabol, 1985, 61(5):873-6.
- Kerlan V, Nahoul K, Le Martelot MT, et al, “Longitudinal Study of Maternal Plasma Bioavailable Testosterone and Androstanediol Glucuronide Levels During Pregnancy,” Clin Endocrinol (Oxf), 1994, 40(2):263-7.
- Davis SR and Burger HG, “Use of Androgens in Postmenopausal Women,” Curr Opin Obstet Gynecol, 1997, 9(3):177-80 (review).
- Jassal SK, Barret-Connor E, and Edelstein SL, “Low Bioavailable Testosterone Levels Predict Future Height Loss in Postmenopausal Women,” J Bone Miner Res, 1995, 10(4):650-4.
- Morley JE, Kaiser F, Raum WJ, et al, “Potentially Predictive and Manipulable Blood Serum Correlates of Aging in the Healthy Male: Progressive Decreases in Bioavailable Testosterone, Dehydroepiandrosterone Sulfate, and the Ratio of Insulin-Like Growth Factor 1 to Growth Hormone,” Proc Natl Acad Sci USA, 1997, 94(14):7537-42.
- Nahoul K and Roger M, “Age-Related Decline of Plasma Bioavailable Testosterone in Adult Men,” J Steroid Biochem Mol Biol, 1990, 35(2):293-9.
- Korenman SG, Morley JE, Mooradian AD, et al, “Secondary Hypogonadism in Older Men: Its Relation to Impotence,” J Clin Endocrinol Metab, 1990, 71(4):963-9
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| References | | Pardridge WM and Mietus LJ, “Transport of Steroid Hormones Through the Rat Blood-Brain Barrier. Primary Role of Albumin-Bound Hormone,” J Clin Invest, 1979, 64(1):145-54. |
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