Background We report detailed results of The Physical Function Trial (PFT), one of seven Testosterone Trials (TTrials), which determined testosterone’s effects on mobility, self-reported physical function, falls, and patient global impression-of-change (PGIC) in older men with self-reported mobility limitation and walking speed<1.2 m/sec. We determined if testosterone’s effects on mobility differed according to baseline walking speed, mobility-limitation, or other participant-level factors. Methods The participants were 788 men≥65 years, with total testosterone<275 ng/dL, of which 390 men with mobility limitation and walking speed<1.2 m/sec enrolled in the PFT. Participants were assigned double-blind to 1% testosterone gel or placebo gel daily for 12-months. Primary outcome was Increase in 6-minute walk distance (6MWD) of ≥50 m; secondary outcomes included absolute increase in 6MWD, physical component of Short Form-36 (PF10), and exploratory outcomes PGIC and falls. Findings Intervention groups were similar at baseline. 6MWD improved significantly more in testosterone than in placebo group among all men in TTrials, and separately in men who were not enrolled in the PFT (treatment effect 8.9, 95% CI (2.2,15.6) p=0.01), but not in those who were enrolled in the PFT (treatment effect 4.1, 95% CI (−3.0,11.2) p=0.25). PF10 improved more in testosterone than in placebo group in all men in TTrials (treatment effect 3.1, 95% CI (1.2, 4.9), p=0.002) and separately in both men enrolled and not enrolled in the PFT (treatment effect 2.8, 95% CI (0.41, 5.2), p=0.02; and treatment effect 4.0, 95% CI (1.5, 6.5), p=0.002, respectively). Testosterone-treated men with baseline walking speed ≥1.2 m/sec experienced significantly greater improvements in 6MWD and in PF10 than placebo-treated men. Men reporting mobility limitation showed significantly more improvement in 6MWD and in PF10 than placebo-treated men. Fall frequency was similar in the two groups. Changes in 6MWD were significantly associated with changes in testosterone, free testosterone, DHT, and hemoglobin levels. Interpretation Testosterone consistently improved self-reported walking ability, modestly improved 6MWD in all men participating in the Testosterone Trials, but did not affect falls. Testosterone’s effect on mobility measures in older men with low testosterone were related to baseline gait speed and self-reported mobility limitation, and changes in testosterone and hemoglobin levels. Trial Registration ClinicalTrials.gov number,

Testosterone treatment of 1 year in older men with low testosterone was associated with small reductions in cholesterol and insulin but not with other glucose markers, markers of inflammation or fibrinolysis, or troponin. The clinical importance of these findings is unclear and requires a larger trial of clinical outcomes.

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