There are usually feedback regulation mechanismsbetween hormones and their target cell products and even other hormones. Someendocrine glands, such as the pituitary and adrenal glands, are closely linkedto the nervous system. The endocrine glands include the pituitary, adrenals,thyroid, parathyroids, pancreas, ovaries, testes, and thymus. Eachadrenal gland is subdivided histologically into a cortex and medulla. The thyroid gland, located in the anteriorneck region, manufactures thyroid gland hormones T3, T4, and calcitonin.
Calcitonin lowers blood calcium and increases bonedensity.
The ovaries, in addition to preparing eggsfor fertilization, secrete estrogen and progesterone. Testes The male testes, in addition to making spermcells, secrete testosterone. The thymus gland is located in the thorax andneck, superior to the heart. This gland secretes thymosin and is involved withprocessing of immune system cells early in body development. Some hormones are secreted by individualcells instead of glands.