Testosterone Replacement Therapy (TRT) is a form of hormone replacement therapy that is used to treat men with low testosterone levels. It involves the administration of testosterone, either through injections, patches, gels, or pellets, to increase the body's testosterone levels. TRT is used to treat a variety of conditions, including hypogonadism, delayed puberty, and andropause (male menopause). It can also be used to improve sexual function, muscle mass, and bone density.