Hormones are special chemicals released by glands in the body to control virtually all bodily processes. They act like messengers telling the different organs and parts of the body how to react or what to do. Sometimes blood levels of a particular hormone can become too high, usually as a result of another medical condition. Medicines can be taken to lower the levels of the hormone or to block its effects. For example, carbimazole and propylthiouracil are used to treat an overactive thyroid gland and bromocriptine blocks the production of the hormone prolactin.