Several. Imbalance of several hormones can cause sexual dysfunction. Deficiency or excess of thyroid hormone, Insulin deficiency (diabetes) deficiency of testosterone and abnormality of other pituitary and adrenal hormones can all effect sexual function in men. In women the same hormones and ovarian hormones can cause sexual dysfunction and loss of libido.
Lack or low hormone. Levels is actually more common in this problem. All of the ones already mentioned are important, but at your age and as a male you should have a pretty good sex drive. If you don't, in addition to the things mentioned, you should have your testosterone and prolactin levels checked. Good luck!