Testosterone replacement therapy is used to treat hypogonadism in males, where the pituitary glands or testicles do not make enough testosterone and males have unwanted conditions such as sexual dysfunction, lower bone and muscle mass, reduced energy and low mood. Testosterone replacement therapy is also known as Low-T therapy, or simply testosterone therapy. It involves giving men synthetic hormones, commonly in the form of injection, cream, or gel. Some men use testosterone patches and oral medications, as well as subcutaneous implants.