Secretin

Secretin is a peptide hormone that regulates water balance throughout the body. It is produced in secretory glands of the duodenum. In humans, the secretin peptide is encoded by the SCT gene, which sits on chromosome 11. 
In 2007, secretin was discovered to play a role in osmoregulation by acting on the hypothalamus, pituitary gland, and kidney.
Secretin was the first hormone to be identified.
The crucial experiments were done at University College London in 1902. The term 'hormone' was invented in 1905.