In most cases low testosterone is caused by the natural decrease of testosterone that occurs as a man ages. There are 2 classifications when referring to Low Testosterone, Primary Hypogonadism and Secondary Hypogonadism. Primary Hypogonadism can be inherited, or acquired by an accident or serious illness. Secondary Hypogonadism is a cause of a damaged hypothalamus or pituitary gland, normal aging, medications, severe stress and even obesity. Most often, there is a mix of Primary and Secondary Hypogonadism.