Low measured free testosterone is associated with worse sexual function in community-dwelling men

OBJECTIVE: Lower free testosterone (T) appears more strongly associated with male sexual dysfunction than total T, but most evidence is based on calculated free T (cFT) rather than measured free T (mFT). This study examined associations between mFT and sexual function in community-dwelling men. DESIGN: Cross-sectional analysis of 526 European Male Ageing Study participants. METHODS: Sexual function was assessed by validated questionnaire. mFT was measured by liquid chromatography tandem mass spectrometry (LC-MS/MS) following equilibrium dialysis. Vermeulen-derived cFT was calculated from LC-MS/MS total T and sex-hormone binding globulin. Associations of sexual outcomes with total T, cFT and mFT were analysed using proportional ordinal logistic regression and restricted cubic splines adjusted for age, body mass index (BMI) and centre. RESULTS: Participants had a mean age 70.5 ± 8.5 years, BMI 27.3 ± 3.3 kg/m2, cFT 269.2 ± 86.6 pmol/L and mFT 198 ± 82 pmol/L. Lower mFT was significantly associated with erectile dysfunction (odds ratio (OR) per 10 pmol/L decrease 1.04[1.01-1.06]), fewer sexual intercourse (OR 1.03[1.00-1.06]), lower orgasm frequency (OR 1.03[1.01-1.06]) and reduced sexual desire (OR 1.03[1.01-1.05]). Total T was not associated with any outcome and cFT only with erectile dysfunction (OR 1.03[1.01-1.05]) and reduced sexual desire (OR 1.03[1.00-1.05]). Non-linear associations were observed for mFT with erectile function and orgasm frequency, with markedly steeper increase in predicted probability below 163 and 138 pmol/L, respectively. CONCLUSION: In community-dwelling men, lower mFT was associated with worse erectile function, fewer sexual intercourse attempts, reduced orgasm frequency, and lower sexual desire, with apparent non-linear thresholds for erectile dysfunction and orgasm frequency at 163 and 138 pmol/L, respectively.