The purpose of this study is to: Address 3 short term objectives; (1) Determine the effects of physiologic testosterone (T) therapy on neurological function and functional independence following traumatic brain injury (TBI) in hypogonadal men during inpatient rehabilitation; (2) Document the natural history of neuroendocrine dysfunction and recovery in men during inpatient rehabilitation after TBI; (3) Obtain data to validate the NIH toolbox, a novel assessment of neurological function for use in the TBI population; and two long-term objectives: (1) Utilize study findings to design a multicenter trial to further assess the impact of T therapy in hypogonadal men following TBI and (2) Impact TBI practice management with new information about neuroendocrine dysfunction after TBI and hormone treatments to improve outcomes.