Testosterone, Free and Weakly Bound

CPT: 84410
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  • Bioavailable Testosterone
  • Free and Albumin-Bound Testosterone

Test Includes

Total testosterone

Special Instructions

This panel includes Testosterone, Total, which may exhibit interference when sample is collected from a person who is consuming a supplement with a high dose of biotin (also termed as vitamin B7 or B8, vitamin H, or coenzyme R). It is recommended to ask all patients who may be indicated for this test about biotin supplementation. Patients should be cautioned to stop biotin consumption at least 72 hours prior to the collection of a sample.

Expected Turnaround Time

1 - 6 days

Related Documents

Specimen Requirements




2.3 mL


Red-top tube or gel-barrier tube


If a red-top tube is used, transfer separated serum to a plastic transport tube.

Storage Instructions

Room temperature

Stability Requirements



Room temperature

14 days


14 days

Causes for Rejection

Gross lipemia; recently administered isotopes

Test Details


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


Ammonium sulfate precipitation; radioassay

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

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.


1. Pardridge WM. Transport of protein-bound hormones into tissues in vivo. Endocr Rev. 1981 Winter; 2(1):103-123 (review). 7028469
2. Cumming DC, Wall SR. Nonsex hormone-binding globulin-bound testosterone as a marker for hyperandrogenism. J Clin Metabol. 1985 Nov; 61(5):873-876. 4044776
3. Kerlan V, Nahoul K, Le Martelot MT, Bercovici JP. Longitudinal study of maternal plasma bioavailable testosterone and androstanediol glucuronide levels during pregnancy. Clin Endocrinol (Oxf). 1994; 40(2):263-267. 8137527
4. Davis SR, Burger HG. Use of androgens in postmenopausal women. Curr Opin Obstet Gynecol. 1997 Jun; 9(3):177-180 (review). 9263701
5. Jassal SK, Barrett-Connor E, Edelstein SL. Low bioavailable testosterone levels predict future height loss in postmenopausal women. J Bone Miner Res. 1995 Apr; 10(4):650-654. 7610937
6. Morley JE, Kaiser F, Raum WJ, et al. Potentially predictive and manipulable blood serum correlates of aging in the healthy human 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 Jul 8; 94(14):7537-7542. 9207127
7. Nahoul K, Roger M. Age-related decline of plasma bioavailable testosterone in adult men. J Steroid Biochem Mol Biol. 1990 Feb; 35(2):293-299. 2106599
8. Korenman SG, Morley JE, Mooradian AD, et al. Secondary hypogonadism in older men: Its relation to impotence. J Clin Endocrinol Metab. 1990 Oct; 71(4):963-969.2205629


Pardridge WM, Mietus LJ. Transport of steroid hormones through the rat blood-brain barrier. Primary role of albumin-bound hormone. J Clin Invest. 1979 Jul; 64(1):145-154.447850


Order Code Order Code Name Order Loinc Result Code Result Code Name UofM Result LOINC
143255 Testosterone,Free+Weakly Bound 58716-2 004227 Testosterone ng/dL 2986-8
143255 Testosterone,Free+Weakly Bound 58716-2 143257 Testost., % Free+Weakly Bound % 6891-6
143255 Testosterone,Free+Weakly Bound 58716-2 143260 Testost., F+W Bound ng/dL 2990-0

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