Hormones serve as messengers to coordinate activities of various parts of the body.
The major organs of the endocrine system are the hypothalamus, the pituitary gland, the thyroid gland, the parathyroid glands, the islets of the pancreas, the adrenal glands, the testes, and the ovaries. During pregnancy, the placenta also acts as an endocrine gland in addition to its other functions. The pituitary gland is sometimes called the master gland because it coordinates many functions of the other endocrine glands. The pituitary controls the rate at which it secretes its own hormones through a feedback loop in which the blood levels of other endocrine hormones signal the pituitary to slow down or speed up. Parathyroid cells respond to calcium and phosphate.