This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Lowering testosterone in men to pre-pubertal levels causes adverse effects on many body functions. Specifically, men with very low testosterone levels experience bone loss, decreased muscle strength, decreased muscle size, increased fat mass, lower sexual drive, and generally feel less well than normal men. As men age, testosterone levels decline gradually (though they rarely become extremely low) and men often experience changes in testosterone-related functions. Still, the degree to which the natural reduction of testosterone may be responsible for these adverse effects is unknown. The goal of this study is to determine the level of testosterone at which these changes begins to occur. This study will enroll 150 healthy men ages 20-50. Subjects will be given an injection every 4 weeks that reversible lowers their testosterone to pre-pubertal levels. (Note that when the injections are stopped, testosterone levels return promptly to normal). Each subject is also given a controlled amount of replacement testoererone gel. There are 5 possible doses of replacement testosterone that are designed to produce blood testosterone levels that range from very low to normal. The dose given to each patient is randomly determined. Over the course of 16 weeks a number of measures will be tracked including bone breakdown, muscle size and strength, fat mass, sexual drive, energy levels, and mood levels. By determining what levels of testosterone are associated with changes in these measures, clinicians may have a better idea when to treat aging men with testosterone.
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