Thesis (M.S.M.) PLEASE NOTE: Boston University Libraries did not receive an Authorization To Manage form for this thesis or dissertation. It is therefore not openly accessible, though it may be available by request. If you are the author or principal advisor of this work and would like to request open access for it, please contact us at open-help@bu.edu. Thank you. / Although testosterone therapy consistently increases hematocrit levels across numerous clinical trials, the factors that affect the relationship between testosterone therapy and on-treatment hematocrit are poorly understood. Here, the effect of testosterone on hematocrit levels was contrasted across subgroups of men from 2 randomized, placebo-controlled trials involving testosterone gel therapy: the Testosterone and Erectile Dysfunction (TED; ClinicalTrials.gov # NCT00512707; n = 130) and the Testosterone in Older Men (TOM; ClinicalTrials.gov # NCT00240981; n = 152). Hematocrit elevations with testosterone therapy were greater among men self-identifying as white (difference between white and black men 2.32, 95% CI 1.03, 3.61; P = 0.0005). Compliance to therapy also modified the effect of testosterone on hematocrit (difference between participants with 99 to 100% vs. 21 to 87% compliance with therapy 1.83%; 95% CI 0.23, 3.42; P = 0.026). On-treatment hematocrit levels were greater among older men (difference between 75 to 87 and 50 to 57 years 1.66%; 95% CI 0.08, 3.24; P = 0.039) and men with hypertension (difference between men with and without hypertension 1.31; 95% CI 0.14, 2.48; P = 0.028). On-treatment hematocrit levels were smaller among men who smoke(treatment effect difference between smokers and non-smokers -1.22%; 95% CI -2.80, 0.36; P = 0.129), men with Chronic Obstructive Pulmonary Disorder (difference - 2.92%; 95% CI -6.41, 0.58; P = 0.101), and those with sleep apnea (difference - 0.88; 95% CI -2.93, 1.16; P = 0.40), however the numbers of men reporting these characteristics are too small to draw strong conclusions. In conclusion, greater increases in hematocrit levels with testosterone therapy may associate with greater compliance, having hypertension, advanced age, and white race. / 2031-01-01