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 . 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
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 . 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 , 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 . 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 . 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
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