The gonads — testes in boys and ovaries in girls — produce most of the body’s sex hormones. When the gonads produce too little hormone (hypogonadism) or too much hormone (hypergonadism), health problems can occur. In girls, hypogonadism during childhood stops menstruation and breast development from occurring, and slows growth. In boys, too little sex hormone during childhood slows development of muscles and facial hair, and results in short stature. In both boys and girls, too much of the hormones can speed sexual development.