Glands that signal each other in sequence are often referred to as an axis, such as the hypothalamic–pituitary–adrenal axis.

The study of the endocrine system and its disorders is known as endocrinology. One of the most important functions of the hypothalamus-it is located in the brain adjacent to the pituitary gland-is to link the endocrine system to the nervous system via the pituitary gland.) Other organs, such as the kidneys, also have roles within the endocrine system by secreting certain hormones. (The hypothalamus and pituitary glands are organs of the neuroendocrine system. The hypothalamus, pancreas, and thymus also function as endocrine glands, among other functions. In humans, the major endocrine glands are the thyroid, parathyroid, pituitary, pineal, and adrenal glands, and the (male) testicles and (female) ovaries. In vertebrates, the hypothalamus is the neural control center for all endocrine systems.

The endocrine system is a messenger system in an organism comprising feedback loops of hormones that are released by internal glands directly into the circulatory system and that target and regulate distant organs.