This study explores age-specific reference ranges for free testosterone (FT) in adult men, offering crucial insights for diagnosing testosterone deficiency. While testosterone levels are known to decrease with age, the clinical utility of this information has been limited due to the lack of age-adjusted reference ranges. Using data from the 2013-2016 National Health and Nutrition Examination Survey, researchers analyzed FT levels in a representative sample of U.S. men aged 20 and older who met specific health criteria. Exclusions applied to men with BMI over 30, elevated serum creatinine, high liver enzymes (AST or ALT), history of diabetes, testicular cancer, thyroid disease, or use of hormonal medications. FT levels were calculated based on total testosterone, sex hormone-binding globulin, and albumin, with age-specific categories separated by 10-year intervals, except for men 80 and older, grouped together.
The study’s final cohort included 1,147 men. Findings revealed substantial individual variability in FT levels within each age category, with the highest average FT concentration of 114.3 pg/mL observed in men in their 20s. A steady decline was noted across age categories, with FT levels dropping by an average of 12.7% per decade, reaching a mean of 45.8 pg/mL in men aged 80 and above. The most significant decrease occurred between the 20s and 30s age groups, with a 16.9% average drop. These results underscore a consistent pattern of declining FT with age and highlight the necessity of more comprehensive, prospective studies to pinpoint other factors that contribute to this decline. This research provides valuable groundwork for developing clinical guidelines, ultimately aiding healthcare providers in assessing and diagnosing testosterone deficiency in adult men with greater accuracy.
