Human chorionic gonadotropin (HCG) is a gonad-stimulating polypeptide hormone. In males, HCG stimulates testosterone production. In hormone replacement therapy HCG helps prevent the side effects of testicular shrinkage that may occur. When your bodies receive testosterone from through hormone replacement therapy, HCG helps keeps your body continuing testosterone production. HCG also assist in increasing adrenal function which can lead to improved libido and increased energy.