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.
Bioidentical hormone replacement therapy
Bioidentical hormones were first used for menopausal symptom relief in the 1930s, after Canadian researcher James Collip developed a method to extract an orally active estrogen from the urine of pregnant women and marketed it as the active agent in a product called Emmenin. It was supplanted on the market when its manufacturer, Ayerst (later Wyeth Pharmaceuticals), began producing the more easily manufactured conjugated equine estrogens in 1941 under the brand name Premarin; by 1992, Premarin was the most widely prescribed drug in the United States.