The endocrine system is the collection of glands that produce hormones that regulate metabolism, growth and development, tissue function, sexual function, reproduction, sleep, and mood. In human beings, the major endocrine glands are the thyroid gland and the adrenal glands. In all vertebrates, the hypothalamus is the neural control center for all endocrine systems. Several glands that signal each other successively are usually referred to as axis. Besides, many other organs that are part of other body systems, including bone, pancreas, kidney, liver, heart, and gonads, have secondary endocrine functions.