There is no consensus on the exact level of testosterone that defines hypogonadism, but in healthy young men, the lower limit of normal for total testosterone (TT) tends to be 280-300 ng/dl (9.8-10.4 nmol/liter) and for free testosterone (FT), 5–9 ng/dl (0.17–0.31 nmol/liter) by equilibrium dialysis [7]. Recent studies using the liquid chromatography tandem mass spectrometry (LC-MS/MS) assay for the measurement of TT concentrations in early morning samples have reported a prevalence of 10-14% in community-dwelling men 65 years of age and older [1-5]. TT represents the sum of FT and testosterone that is bound to plasma proteins including albumin and
sex hormone-binding globulin (SHBG).
As TT concentrations are affected by the circulating concentrations of SHBG, measurement of FT levels is important in conditions in which alterations in binding protein concentrations may be expected. Free testosterone measurements by analog methods have consistently been found to be inaccurate and are not recommended [8]. Instead, the Endocrine Society advises measuring FT directly by equilibrium dialysis or using a law-of-mass-action equation to derive a calculated FT (cFT) from TT, SHBG, and albumin [7, 9]. These equations employ the currently used model of testosterone’s binding to SHBG, which assumes that each SHBG dimer binds two testosterone molecules and that the two binding sites on SHBG have similar binding affinity [9-11]. However, cFT values using this method may be different from those obtained by the gold standard equilibrium dialysis, and different equations often yield discrepant results [12, 13].
Gender and SHBG concentration affect the accuracy of the cFT [13], and genetic variants in the SHBG locus have recently been identified, which affect testosterone binding to SHBG as well as the FT fraction, possibly invalidating the cFT obtained via the law-of-mass-action equations in those cases [14]. cFT values obtained using a new, multi-step, dynamic, allosteric model of testosterone’s binding to sex hormone binding globulin (SHBG) do have close correspondence with those measured using equilibrium dialysis, but this is not readily available to the public yet [12]. Identification and treatment of symptomatic hypogonadism can help to prevent long-term consequences of accompanying co-morbidities while improving a patient’s quality of life; however, the diagnosis should be based on the determination of signs and symptoms of androgen deficiency along with unequivocally low levels of testosterone. It is unclear at this time whether FT or TT is a better test to diagnose and treat androgen deficiency but what is clear is that clinicians must have reliable assays and algorithms based on accurate binding models before making the diagnosis of hypogonadism and initiating treatment.
Written by: Anna Ross, MD
Research Program in Men's Health: Aging and Metabolism, Brigham and Women's Hospital, 221 Longwood Avenue, Boston, MA 02115, USA
Read the Abstract
References
1. Araujo, A.B., et al., Prevalence of symptomatic androgen deficiency in men. J Clin Endocrinol Metab, 2007. 92(11): p. 4241-7.
2. Bhasin, S., et al., Reference ranges for testosterone in men generated using liquid chromatography tandem mass spectrometry in a community-based sample of healthy nonobese young men in the Framingham Heart Study and applied to three geographically distinct cohorts. J Clin Endocrinol Metab. 96(8): p. 2430-9.
3. Hans, D., et al., Correlations between trabecular bone score, measured using anteroposterior dual-energy X-ray absorptiometry acquisition, and 3-dimensional parameters of bone microarchitecture: an experimental study on human cadaver vertebrae. J Clin Densitom. 14(3): p. 302-12.
4. Orwoll, E., et al., Testosterone and estradiol among older men. J Clin Endocrinol Metab, 2006. 91(4): p. 1336-44.
5. Wu, F.C., et al., Identification of late-onset hypogonadism in middle-aged and elderly men. N Engl J Med. 363(2): p. 123-35.
6. Trost, L.W. and J.P. Mulhall, Challenges in Testosterone Measurement, Data Interpretation, and Methodological Appraisal of Interventional Trials. J Sex Med. 13(7): p. 1029-46.
7. Bhasin, S., et al., Testosterone therapy in men with androgen deficiency syndromes: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 95(6): p. 2536-59.
8. Rosner, W., et al., Position statement: Utility, limitations, and pitfalls in measuring testosterone: an Endocrine Society position statement. J Clin Endocrinol Metab, 2007. 92(2): p. 405-13.
9. Vermeulen, A., L. Verdonck, and J.M. Kaufman, A critical evaluation of simple methods for the estimation of free testosterone in serum. J Clin Endocrinol Metab, 1999. 84(10): p. 3666-72.
10. Mazer, N.A., A novel spreadsheet method for calculating the free serum concentrations of testosterone, dihydrotestosterone, estradiol, estrone and cortisol: with illustrative examples from male and female populations. Steroids, 2009. 74(6): p. 512-9.
11. Sodergard, R., et al., Calculation of free and bound fractions of testosterone and estradiol-17 beta to human plasma proteins at body temperature. J Steroid Biochem, 1982. 16(6): p. 801-10.
12. Zakharov, M.N., et al., A multi-step, dynamic allosteric model of testosterone's binding to sex hormone binding globulin. Mol Cell Endocrinol. 399: p. 190-200.
13. Hackbarth, J.S., et al., Accuracy of calculated free testosterone differs between equations and depends on gender and SHBG concentration. Steroids. 76(1-2): p. 48-55.
14. Ohlsson, C., et al., Genetic determinants of serum testosterone concentrations in men. PLoS Genet. 7(10): p. e1002313.
As TT concentrations are affected by the circulating concentrations of SHBG, measurement of FT levels is important in conditions in which alterations in binding protein concentrations may be expected. Free testosterone measurements by analog methods have consistently been found to be inaccurate and are not recommended [8]. Instead, the Endocrine Society advises measuring FT directly by equilibrium dialysis or using a law-of-mass-action equation to derive a calculated FT (cFT) from TT, SHBG, and albumin [7, 9]. These equations employ the currently used model of testosterone’s binding to SHBG, which assumes that each SHBG dimer binds two testosterone molecules and that the two binding sites on SHBG have similar binding affinity [9-11]. However, cFT values using this method may be different from those obtained by the gold standard equilibrium dialysis, and different equations often yield discrepant results [12, 13].
Gender and SHBG concentration affect the accuracy of the cFT [13], and genetic variants in the SHBG locus have recently been identified, which affect testosterone binding to SHBG as well as the FT fraction, possibly invalidating the cFT obtained via the law-of-mass-action equations in those cases [14]. cFT values obtained using a new, multi-step, dynamic, allosteric model of testosterone’s binding to sex hormone binding globulin (SHBG) do have close correspondence with those measured using equilibrium dialysis, but this is not readily available to the public yet [12]. Identification and treatment of symptomatic hypogonadism can help to prevent long-term consequences of accompanying co-morbidities while improving a patient’s quality of life; however, the diagnosis should be based on the determination of signs and symptoms of androgen deficiency along with unequivocally low levels of testosterone. It is unclear at this time whether FT or TT is a better test to diagnose and treat androgen deficiency but what is clear is that clinicians must have reliable assays and algorithms based on accurate binding models before making the diagnosis of hypogonadism and initiating treatment.
Written by: Anna Ross, MD
Research Program in Men's Health: Aging and Metabolism, Brigham and Women's Hospital, 221 Longwood Avenue, Boston, MA 02115, USA
Read the Abstract
References
1. Araujo, A.B., et al., Prevalence of symptomatic androgen deficiency in men. J Clin Endocrinol Metab, 2007. 92(11): p. 4241-7.
2. Bhasin, S., et al., Reference ranges for testosterone in men generated using liquid chromatography tandem mass spectrometry in a community-based sample of healthy nonobese young men in the Framingham Heart Study and applied to three geographically distinct cohorts. J Clin Endocrinol Metab. 96(8): p. 2430-9.
3. Hans, D., et al., Correlations between trabecular bone score, measured using anteroposterior dual-energy X-ray absorptiometry acquisition, and 3-dimensional parameters of bone microarchitecture: an experimental study on human cadaver vertebrae. J Clin Densitom. 14(3): p. 302-12.
4. Orwoll, E., et al., Testosterone and estradiol among older men. J Clin Endocrinol Metab, 2006. 91(4): p. 1336-44.
5. Wu, F.C., et al., Identification of late-onset hypogonadism in middle-aged and elderly men. N Engl J Med. 363(2): p. 123-35.
6. Trost, L.W. and J.P. Mulhall, Challenges in Testosterone Measurement, Data Interpretation, and Methodological Appraisal of Interventional Trials. J Sex Med. 13(7): p. 1029-46.
7. Bhasin, S., et al., Testosterone therapy in men with androgen deficiency syndromes: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 95(6): p. 2536-59.
8. Rosner, W., et al., Position statement: Utility, limitations, and pitfalls in measuring testosterone: an Endocrine Society position statement. J Clin Endocrinol Metab, 2007. 92(2): p. 405-13.
9. Vermeulen, A., L. Verdonck, and J.M. Kaufman, A critical evaluation of simple methods for the estimation of free testosterone in serum. J Clin Endocrinol Metab, 1999. 84(10): p. 3666-72.
10. Mazer, N.A., A novel spreadsheet method for calculating the free serum concentrations of testosterone, dihydrotestosterone, estradiol, estrone and cortisol: with illustrative examples from male and female populations. Steroids, 2009. 74(6): p. 512-9.
11. Sodergard, R., et al., Calculation of free and bound fractions of testosterone and estradiol-17 beta to human plasma proteins at body temperature. J Steroid Biochem, 1982. 16(6): p. 801-10.
12. Zakharov, M.N., et al., A multi-step, dynamic allosteric model of testosterone's binding to sex hormone binding globulin. Mol Cell Endocrinol. 399: p. 190-200.
13. Hackbarth, J.S., et al., Accuracy of calculated free testosterone differs between equations and depends on gender and SHBG concentration. Steroids. 76(1-2): p. 48-55.
14. Ohlsson, C., et al., Genetic determinants of serum testosterone concentrations in men. PLoS Genet. 7(10): p. e1002313.