The endocrine system is the various glands in the human body that secrete hormones directly into the circulatory system (the blood stream). The endocrine system consists of the pineal gland, the pituitary gland, the pancreas, the ovaries, the testes, the thyroid, the parathyroid, the hypothalamus, the gastrointestinal tract, and the adrenal glands.
Three endocrine glands are in the human brain. They are the hypothalamus, the pineal gland, and the pituitary gland, while the thyroid is in the neck. These glands release a number of vital hormones including dopamine, somatostatin, oxytocin, and vasopressin from the hypothalamus, melatonin from the pineal, and several growth hormones from the pituitary. The thyroid releases triiodothyrone as its main hormone, which increases metabolic rate and promotes protein synthesis.
Several of the body's major organs also have secondary endocrine functions. The kidneys, liver and heart secret various hormones vital to health bodily regulation. The pancreas is a mixed endocrine and exocrine gland as it secrets both hormones and enzymes. The major hormones secreted by the pancreas include insulin and glucagon.
There are, of course, numerous diseases that affect the endocrine system: diabetes, thyroid disease, and obesity being the most common. Endocrine disease occurs through disregulated hormone release, the lack of a gland, or structural enlargement of a gland such as the thyroid.