Primary hypogonadism is caused by testicular dysfunction resulting in low testosterone and sperm production. It can be caused by genetic conditions like Klinefelter syndrome, infections, trauma, or chemotherapy/radiation exposure. Physical exam may show small, soft testes and low body hair with high FSH and LH levels. Treatment is testosterone replacement therapy. Secondary hypogonadism results from pituitary or hypothalamic dysfunction, with proportionate low testosterone and sperm production and low or normal FSH and LH levels.