Secretin is a hormone that regulates water levels in the body by acting on the hypothalamus, pituitary gland and kidneys, and modulates the pH of the duodenum by acting on secretions in the stomach, pancreas and liver. It is produced by the cells of the duodenum. It was the first hormone ever to be identified and was the first to ever be called a "hormone", a term coined by one of its co-discoverers.
When secretin is released, it causes a decrease in the acidity of the material in the duodenum by suppressing the production of stomach acid. However, when the acidity of the duodenum reaches a certain level, secretin can no longer be released and the acidity increases again, forming a negative feedback loop. The same hormone also urges the pancreas to create bicarbonate and insulin. The bicarbonate again lowers the acidity of the material as it exits the duodenum to a level that is comfortable for the small intestine. The insulin is released into the bloodstream to regulate blood sugar levels as glucose is released by digestion. Keeping the duodenum within a narrow pH range optimizes the action of the enzymes in this part of the digestive tract.
Simultaneously, secretin opens pathways for water molecules so they can be absorbed into the body from the water content of food, and also triggers the kidneys to increase urination in response.
Pharmaceutical preparations of secretin are used to test pancreas function.