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Free testosterone is the concentration of unbound testosterone in serum.1 The majority (approximately 60% to 90%) of serum total testosterone is associated with sex hormone binding globulin (SHBG); this fraction is tightly bound and biologically unavailable to its target tissues. The remaining bioavailable testosterone is mostly bound to albumin, with only a small fraction (approximately 0.5% to 2%) circulating in the free form.1,2 Free testosterone is the form of testosterone that can diffuse into the tissues and act on receptors and is considered the active fraction by many physicians.1,3,4 Free testosterone assessment is recommended as a primary or secondary measure of androgen activity in men2,5-11 and women.12-16 The equilibrium dialysis or ultrafiltration methods are recommended, however they are relatively difficult and expensive to perform. All of these methods are currently available from LabCorp.
The primary screening test for the diagnosis of hypoandrogenism in men is the measurement of total testosterone in serum in a morning sample.2,5-11 Low concentrations of testosterone in serum should be confirmed by repeat measurement, preferably using liquid chromatography/mass spectrometry.6,7,10 Determination of free testosterone can be of value in men with borderline total testosterone because alterations in SHBG levels can markedly affect the concentration of biologically available free testosterone.2,17-20 Factors and conditions that tend to increase SHBG concentrations in men include aging, hyperthyroidism, estrogens, HIV disease, anticonvulsant therapy, and liver disease.2 Factors and conditions that tend to decrease SHBG concentrations in men include obesity, diabetes mellitus, hypothyroidism, glucocorticoid therapy, androgenic steroid therapy, nephrotic syndrome and acromegaly.2
The binding affinities between testosterone, SHBG and albumin have been measured experimentally, and it has been shown that if the concentrations of serum total testosterone, SHBG, and albumin are measured, the corresponding concentration of free testosterone can be accurately estimated according to equilibrium kinetics.21 The widely accepted method of calculation of serum free testosterone is based upon a 1999 paper by Vermeulen.21 The authors derived the equation based upon the law of mass action. Using the known association constants between testosterone, SHBG, and albumin, they calculated the theoretically predicted serum free testosterone concentrations. This method demonstrated substantial agreement with equilibrium dialysis measured concentrations when testosterone, SHBG and albumin are measured using reliable assays.
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This test was developed and its performance characteristics determined by LabCorp. It has not been cleared or approved by the Food and Drug Administration.
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2. Paduch DA, Brannigan RE, Fuchs EF, Kim ED, Marmar JL, Sandlow JI. The laboratory diagnosis of testosterone deficiency. Urology. 2014 May; 83(5):980-988. PubMed 24548716
3. Krasnoff JB, Basaria S, Pencina MJ, et al. Free testosterone levels are associated with mobility limitation and physical performance in community-dwelling men: the Framingham Offspring Study. J Clin Endocrinol Metab. 2010 Jun;95(6):2790-2799. PubMed 20382680
4. Shea JL, Wongt PY, Chen Y. Free testosterone: clinical utility and important analytical aspects of measurement. Adv Clin Chem. 2014; 63:59-84. PubMed 24783351
5. Bhasin S, Cunningham GR, Hayes FJ, et al. Testosterone therapy in men with androgen deficiency syndromes: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2010 Jun;95(6):2536-2559. PubMed 20525905
6. Bhasin S, Basaria S. Diagnosis and treatment of hypogonadism in men. Best Pract Res Clin Endocrinol Metab. 2011 Apr;25(2):251-270. PubMed 21397197
7. Basaria S. Male hypogonadism. Lancet. 2014 Apr 5;383(9924):1250-1263. PubMed 24119423
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9. McGill JJ, Shoskes DA, Sabanegh ES. Androgen deficiency in older men: indications, advantages, and pitfalls of testosterone replacement therapy. Cleve Clin J Med. 2012 Nov;79(11):797-806. PubMed 23125330
10. Wu FC, Tajar A, Beynon JM, et al. Identification of late onset hypogonadism in middle-aged and elderly men. N Engl J Med. 2010 Jul 8;363(2):123-135. PubMed 20554979
11. Wang C, Nieschlag E, Swerdloff R, et al. Investigation, treatment, and monitoring of late-onset hypogonadism in males: ISA, ISSAM, EAU, EAA, and ASA recommendations. Eur Urol. 2009 Jan;55(1):121-130. PubMed 18762364
12. Wierman ME, Arlt W, Basson R, et al. Androgen therapy in women: a reappraisal: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2014 Oct;99(10):3489-3510. PubMed 25279570
13. Mueller A, Dittrich R, Cupisti S, Beckmann MW, Binder H. Is it necessary to measure free testosterone to assess hyperandrogenemia in women? The role of calculated free and bioavailable testosterone. Exp Clin Endocrinol Diabetes. 2006 Apr;114(4):182-187. PubMed 16705550
14. Stanczyk FZ. Diagnosis of hyperandrogenism: biochemical criteria. Best Pract Res Clin Endocrinol Metab. 2006 Jun;20(2):177-191. PubMed 16772150
15. Legro RS, Arslanian SA, Ehrmann DA, et al. Diagnosis and treatment of polycystic ovary syndrome: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2013 Dec;98(12):4565-4592. PubMed 24151290
16. Azziz R, Carmina E, Dewailly D, et al. The Androgen Excess and PCOS Society criteria for the polycystic ovary syndrome: the complete task force report. Fertil Steril. 2009 Feb;91(2):456-488. PubMed 18950759
17. Giannetta E, Gianfrilli D, Barbagallo F, Isidori AM, Lenzi A. Subclinical male hypogonadism. Best Pract Res Clin Endocrinol Metab. 2012 Aug;26(4):539-550 PubMed 22863395
18. Cooper LA, Page ST, Amory JK, Anawalt BD, Matsumoto AM. The association of obesity with sex hormone-binding globulin is stronger than the association with ageing - implications for the interpretation of total testosterone measurements. Clin Endocrinol (Oxf). 2015 Dec;83(6);828-833. PubMed 25777143
19. Guay AT, Traish AM, Hislop-Chestnut DT, Doros G, Gawoski JM. Are there variances of calculated free testosterone attributed to variations in albumin and sex hormone-binding globulin concentrations in men? Endocr Pract. 2013 Mar-Apr;19(2):236-242. PubMed 23543029
20. Anawalt BD, Hotaling JM, Walsh TJ, Matsumoto AM. Performance of total testosterone measurement to predict free testosterone for the biochemical evaluation of male hypogonadism. J Urol. 2012 Apr;187(4):1369-1373. PubMed 22341266
21. Vermeulen A, Verdonck L, Kaufman JM. A critical evaluation of simple methods for the estimation of free testosterone in serum. J Clin Endocrinol Metab. 1999 Oct;84(10):3666-3672. PubMed 10523012
22. Reference intervals for calculated free testosterone developed through LabCorp Internal Studies.
