The endocrine system regulates all chemical reactions in cells. The substances responsible for this regulation are known as hormones. Hormones are produced by endocrine glands. The major endocrine glands are the pituitary, thyroid, parathyroid, adrenal, and pancreas. Once a hormone is released into the bloodstream, it travels to its target tissue and produces a response. An awareness of this system can help medical assistants be more effective when teaching a patient about the advantages and disadvantages of hormone replacement therapy.