The thymus is considered an endocrine gland because it secretes a hormone, thymosin, which stimulates the T lymphocytes of the immune system. The gonads are also included because, in addition to producing the sex cells, they secrete hormones. Other organs, including the stomach, kidney, heart, and small intestine, also produce hormones. However, they have other major functions and are discussed with the systems to which they belong.

Finally, prostaglandins are a group of hormones produced by many cells. They have a variety of effects, including stimulation of uterine contractions, promotion of the inflammatory response, and vasomotor activities. They are called prostaglandins because they were first discovered in the prostate gland.

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