Sociodemographic, lifestyle, and medical factors associated with calculated free testosterone concentrations in men: individual participant data meta-analyses

Objective: Sociodemographic, lifestyle, and medical variables influence total testosterone (T) and sex hormone-binding globulin (SHBG) concentrations. The relationship between these factors and “free” T remains unclear. We examined 21 sociodemographic, lifestyle, and medical predictors influencing calculated free T (cFT) in community-dwelling men across ages. Design: This is a cross-sectional analysis in 20 631 participants in the Androgens in Men Study. Methods: Individual participant data (IPD) were provided by 9 cohorts. Total T was determined using mass spectrometry, SHBG using immunoassays, and cFT using the Vermeulen formula. Associations were analyzed using 2-stage random effects IPD meta-analyses. Results: Cohort median ages ranged from 40 to 76 years and median cFT concentrations from 174.3 to 422.8 pmol/L. In men aged 17-99 years, there was a linear inverse association of cFT with age (−57.2 pmol/L [95% confidence interval, −69.4, −44.9] per 1 SD increase in age). Calculated free T increased with increasing baseline body mass index (BMI) among men with BMI < 23.6 kg/m², but decreased among men with BMI > 23.6 kg/m² (−24.7 pmol/L [−29.1, −20.3] per 1 SD increase in the 25.4-29.6 kg/m² BMI range). Calculated free T was lower in younger men, who were married or in a de facto relationship (−18.4 pmol/L [−27.6, −9.3]) and in men who formerly smoked (−5.7 pmol/L [−8.9, −2.6]), were in poor general health (−14.0 pmol/L [−20.1, −7.8]), and had diabetes (−19.6 pmol/L [−23.0, −16.3]), cardiovascular disease (−5.8 pmol/L [−8.3, −3.2]), or cancer (−19.2 pmol/L [−24.4, −14.1]). Conclusions: Calculated free T was most prominently associated with age and BMI. The linear, inverse association with age, nonlinear association with BMI, and presence of diabetes, cancer, and sociodemographic factors should be considered when interpreting cFT values.