There are usually feedback regulation mechanisms between hormones and their target cell products and even other hormones. Some endocrine glands, such as the pituitary and adrenal glands, are closely linked to the nervous system. The endocrine glands include the pituitary, adrenals, thyroid, parathyroids, pancreas, ovaries, testes, and thymus. Each adrenal gland is subdivided histologically into a cortex and medulla. The thyroid gland, located in the anterior neck region, manufactures thyroid gland hormones T3, T4, and calcitonin.
Calcitonin lowers blood calcium and increases bone density.
The ovaries, in addition to preparing eggs for fertilization, secrete estrogen and progesterone. Testes The male testes, in addition to making sperm cells, secrete testosterone. The thymus gland is located in the thorax and neck, superior to the heart. This gland secretes thymosin and is involved with processing of immune system cells early in body development. Some hormones are secreted by individual cells instead of glands.