A hormone is a chemical made by specialized cells, usually in an endocrine gland, and it's released into the bloodstream to send a message to another part of the body. Since hormones are released into the bloodstream and therefore can be carried throughout the body, they can perform these two actions on many different targets. The complex interplay between glands, hormones, and other target organs is called the endocrine system. Hormones affect many physiological activities, including growth, metabolism, appetite, puberty, and fertility. The best and most popular hormones on the market in 2020 are found on this page.