This study will assess the hormonal status of men on admission to an inpatient TBI rehabilitation program. Eighty-eight Individuals with low Testosterone (T) levels will be randomized to receive either physiologic T therapy or placebo. Fifty-six Individuals with sufficient levels of T will be followed as a second control group. All three groups will undergo serial assessment of neurological function (as measured by the NIH Toolbox) and functional independence (FIM) to correlate with hormone levels. Hormone levels will be assessed through analysis of blood samples drawn from all participants at the time of study screening and at two week intervals for a total of 7 blood draws during the 12 week study period. Demographics, injury characteristics, concomitant medication usage and adverse events will be documented by medical record review. Statistical analysis will be performed to test study hypotheses. Findings will be disseminated at professional and consumer conferences and in peer-reviewed journals and consumer-based publications. Results will also be made available through the Craig Hospital website and via summary report to the Colorado TBI Trust Fund Research Program. If T therapy is found to be effective, findings will be used as preliminary data for a grant proposal for a multi-center trial.