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.
Thyroid
French chemist Bernard Courtois discovered Iodine in 1811, and in 1896 Eugen Baumann documented it as the central ingredient in the thryoid gland. He did this by boiling the thyroid glands of a thousand sheep, and named the precipitate, a combination of the thyroid hormones, 'iodothyrin'. David Marine in 1907 provided iodine is necessary for thyroid function. Thyroxine itself was first isolated in 1914 and synthesized in 1927, and trirodothyroxine in 1952. The conversion of T4 to T3 was discovered in 1970. The process of discovering TSH took place over the early to mid twentieth century. TRH was discovered by Polish endocrinologist Andrew Schally in 1970, contributing in part to his Nobel Prize in Medicine in 1977.