Take the example of the control of thyroid hormones from the thyroid gland in your neck. The hypothalamus produces a hormone called thyrotropin-releasing hormone (TRH). TRH travels to the pituitary gland and stimulates it to release thyroid-stimulating hormone (TSH). TSH travels to the thyroid gland and stimulates it to release thyroid hormones, T3 and T4. When levels of T3 and T4 get high enough, levels in the bloodstream are detected by the hypothalamus. The hypothalamus then knows to stop releasing more TRH. This last part of the loop is called negative feedback and prevents hormone levels from rising too high.
Estrogens -- particularly estradiol -- participate in a large number of vital processes in your body. In addition to their well-known roles in sexual differentiation and reproduction, estrogens are essential for normal brain development during fetal life, and they help support brain function during adulthood. Estrogens help maintain the integrity of your skeleton, skin and blood vessels, and they are instrumental in lipid metabolism. The physiologic necessity of estrogens is demonstrated in the number of organs and tissues that produce and respond to these important hormones.
The arrangement in lampreys , which are among the most primitive of all fish, may indicate how the pituitary originally evolved in ancestral vertebrates. Here, the posterior pituitary is a simple flat sheet of tissue at the base of the brain, and there is no pituitary stalk. Rathke's pouch remains open to the outside, close to the nasal openings. Closely associated with the pouch are three distinct clusters of glandular tissue, corresponding to the intermediate lobe, and the rostral and proximal portions of the anterior pituitary. These various parts are separated by meningial membranes, suggesting that the pituitary of other vertebrates may have formed from the fusion of a pair of separate, but associated, glands.