Secretin
Secretin was the first hormone to be identified. In 1902, William Bayliss and Ernest Starling were studying how the nervous system controls the process of digestion. It was known that the pancreas secreted digestive juices in response to the passage of food (chyme) through the pyloric sphincter into the duodenum. They discovered (by cutting all the nerves to the pancreas in their experimental animals) that this process was not, in fact, governed by the nervous system. They determined that a substance secreted by the intestinal lining stimulates the pancreas after being transported via the bloodstream. They named this intestinal secretion "secretin". Secretin was the first such "chemical messenger" identified. This type of substance is now called a hormone, a term coined by Bayliss in 1905.
Donald F. Steiner
Donald F. Steiner is known for his discoveries on proinsulin, the precursor of insulin which is a hormone that helps the body control the use of sugar. He found that insulin is synthesized from a larger precursor protein in the beta cells of the pancreas and this led the way to elucidation of how the islet cells function, and how peptide hormones are synthesized and metabolized. He and his colleagues discovered some of the enzymes that convert proinsulin into insulin, and also devised methods for measuring insulin and its precursors in human serum.