The endocrine glands provide the body with chemicals called hormones. Once produced, hormones enter the bloodstream and most (other than prostaglandin) produce an effect elsewhere in the body. Not all cells within the body are affected by hormones, and only some cells of a particular organ may respond to a specific hormone.
Some hormones control the release of other hormones. For example, the pituitary gland located at the base of the brain produces many hormones. These hormones act on other glands such as the adrenal glands and cause them to release their own hormones. The pituitary gland is called "the master gland" as it provides more kinds of hormones than any other gland. Pituitary hormones control the hormone release from other endocrine glands, including the thyroid, parathyroid, adrenal, ovaries, testicles, and pancreas.
The pituitary gland produces growth hormone, which controls growth; prolactin, which stimulates the mammary glands to produce milk; and thyroid-stimulating hormone (TSH), which stimulates the thyroid gland. Luteinizing hormone (LH) and follicle-stimulating hormone (FSH) are two hormones produced by the pituitary gland which control heat cycles and ovulation. The pituitary gland also produces adrenocorticotropic hormone (ACTH) which causes the adrenal gland to produce cortisol and other hormones; melanocyte-stimulating hormone (MSH), which affects pigment; and antidiuretic hormone (ADH) which regulates the metabolism of water.
The thyroid gland, once stimulated, produces its own hormone, thyroxine. The ovaries, once stimulated by FSH and LH from the pituitary, principally produce progesterone and estrogens. The testes provide testosterone. The pancreas produces the most well-known hormone of all; insulin, which regulates blood sugar. The adrenal glands, once stimulated by the pituitary hormone, ACTH, produce naturally occurring steroids called corticosteroids, mineralocorticoids, and adrenal sex steroids.
As one can see, hormones play a very complex role in regulating the body's functions.