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Evaluate hirsutism and masculinization in women; evaluate testicular function in clinical states in which the testosterone-binding proteins may be altered (obesity, cirrhosis, thyroid disorders).
This test was developed and its performance characteristics determined by LabCorp. It has not been cleared or approved by the Food and Drug Administration.
The concentration of free testosterone is very low, typically <2% of the total testosterone concentration. In most men and women, >50% of total circulating testosterone is bound to sex hormone-binding globulin (SHBG) and most of the rest is bound to albumin.1 Routinely available assay methods used to measure total testosterone are not sensitive enough to quantitate accurately the free testosterone fraction directly. Free testosterone is estimated in this profile by an indirect method, equilibrium ultrafiltration. Tritiated testosterone is added to the sample and allowed to come to equilibrium with testosterone in the serum at physiological temperature.2,3 The amount of the added radiolabeled testosterone must be low enough to ensure that the addition will not significantly increase the total testosterone concentration. Once equilibrium is achieved, the free testosterone is separated from the bound testosterone by filtration through a membrane by centrifugal ultrafiltration.4 The radioactivity of the protein-free ultrafiltrate is measured and used to calculate the percent free testosterone. The concentration of free testosterone is then calculated by multiplying the percent free testosterone by the total testosterone concentration.
1. Wheeler MJ. The determination of bioavailable testosterone. Ann Clin Biochem. 1995; 32(Pt 4):345-357 (review). PubMed 7486793
2. Klee GG, Heser DW. Techniques to measure testosterone in the elderly. Mayo Clin Proc. 2000; 75(Suppl):S19-S25. PubMed 10959211
3. Gronowski AM, Landau-Levine M. Reproductive endocrine function. In: Burtis CA, Ashwood ER, eds. Tietz Textbook of Clinical Chemistry. 3rd ed. Philadelphia, Pa: WB Saunders Co; 1999:1601-1641.
4. Vlahos I, MacMahon W, Sgoutas D. An improved ultrafiltration method for determining free testosterone in serum. Clin Chem. 1982; 28(11):2286-2291. PubMed 7127776
