Endocrine glands have no ducts, are vascular, and commonly have intracellular vacuoles or granules that store their hormones. The endocrine system can be contrasted to both exocrine glands, which secrete hormones to the outside of the body, and paracrine signalling between cells over a relatively short distance. Hormones can be amino acid complexes, steroids, eicosanoids, leukotrienes, or prostaglandins. For example, the kidney secretes the endocrine hormone erythropoietin. In addition to the specialized endocrine organs mentioned above, many other organs that are part of other body systems have secondary endocrine functions, including bone, kidneys, liver, heart and gonads. 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. Other organs, such as the kidneys, also have roles within the endocrine system by secreting certain hormones.

The hypothalamus, pancreas, and thymus also function as endocrine glands, among other functions. In humans, the major endocrine glands are the thyroid gland, parathyroid gland, pituitary gland, pineal gland, the testes (male), ovaries (female), and the adrenal glands. In vertebrates, the hypothalamus is the neural control center for all endocrine systems. The endocrine system is a messenger system comprising feedback loops of the hormones released by internal glands of an organism directly into the circulatory system, regulating distant target organs.

Hormones produced by the adrenal gland