Background
Different factors modulate circulating testosterone in men, impacting interpretation of reference ranges. By conducting an individual participant data (IPD) meta-analysis of major cohort studies measuring testosterone accurately, we aimed to clarify sex hormone concentrations and factors associated with these in men.
Methods
Systematic literature searches (June-December 2019) identified prospective cohort studies of adult men with total testosterone measured using mass spectrometry. IPD data were requested. Cross-sectional analyses related total testosterone, sex hormone binding globulin (SHBG), luteinising hormone (LH), dihydrotestosterone (DHT) and estradiol concentrations to sociodemographic, lifestyle, and health factors. Summary curves and summary effect estimates with 95% confidence intervals (CIs) were obtained using two-stage random-effects IPD meta-analyses (PROSPERO: CRD42019139668).
Findings
Summary estimates were obtained from 11 studies (25,364 adult men). There was a non-linear association of testosterone with age, with negligible change among 17-70 year olds (per SD increase age -0.27 nmol/L, CI -0.71,0.18) and decreasing testosterone with age for men >70 years (-1.24 nmol/L, CI -1.61,-0.87). Testosterone was inversely associated with BMI (per SD increase BMI -2.42 nmol/L, CI -2.70,-2.13). Testosterone concentrations were lower for men who: were married/de facto (-0.57 nmol/L); undertook ≤75 minutes vigorous physical activity/week (-0.51 nmol/L); former smokers (-0.34 nmol/L); had hypertension (-0.53 nmol/L), cardiovascular disease (-0.35 nmol/L), cancer (-1.39 nmol/L), or diabetes (-1.43 nmol/L); all CIs <0.0. SHBG increased with age and decreased with BMI, while LH increased non-linearly with age. DHT and estradiol were less prominently associated with these factors.
Interpretation
Multiple factors are associated with variation in male testosterone and SHBG concentrations, with evidence of primary impairment of testicular hormone production after age 70 years. Interpretation of testosterone results in individuals needs to account for these factors, especially age >70 years, BMI, diabetes and cancer. Further research is needed to determine health impacts of declining testosterone in older men.