Hormones act as chemical messengers that are released into the bloodstream to act on an organ in another part of the body. Although hormones reach all parts of the body, only target cells with compatible receptors are equipped to respond. Over 50 hormones have been identified in humans and other vertebrates.

Hormones control or regulate many biological processes and are often produced in unusually low amounts in the body. Examples of such processes include:

• blood sugar control (insulin);
• differentiation, growth and function of reproductive organs (testosterone and estradiol); and
• body growth and energy production (growth hormone and thyroid hormone).

Much like a lock and key, many hormones work by binding to receptors produced in cells. When a hormone binds to a receptor, the receptor carries out the hormone’s instructions, either by changing the cell’s existing proteins or by turning on genes to build a new protein. The hormone-receptor complex turns on or off specific biological processes in cells, tissues and organs.

Some examples of hormones include:

• Estrogens are the group of hormones responsible for female sexual development. They are produced primarily by the ovaries and in small amounts by the adrenal glands.
• Androgens are responsible for male sex characteristics. Testosterone, the sex hormone produced by the testicles, is an androgen.
• The thyroid gland secretes two main hormones, thyroxine and triiodothyronine, into the bloodstream. These thyroid hormones stimulate all cells in the body and control biological processes such as growth, reproduction, development and metabolism.

The endocrine system, which consists of all the body’s various hormones, regulates all biological processes in the body from conception through adulthood and into old age, including the development of the brain and nervous system, the growth and function of the reproductive system, as well as metabolism and blood sugar levels. The female ovaries, male testicles, and the pituitary gland, thyroid gland, and adrenal glands are major components of the endocrine system.

Hypothalamus – The hypothalamus connects our endocrine and nervous systems. The hypothalamus runs the endocrine system.

The pituitary gland – The pituitary receives signals from the hypothalamus. This gland has two lobes, the posterior and the anterior lobes. The posterior lobe secretes hormones that are produced by the hypothalamus. The anterior lobe produces its own hormones, several of which act on other endocrine glands.

Thyroid gland – The thyroid gland is essential for the healthy development and maturation of vertebrates and regulates the metabolism.

Adrenal glands – The adrenal gland consists of two glands: the cortex and the medulla. These glands produce hormones in response to stress and regulate blood pressure, glucose metabolism and the body’s salt and water balance.

Pancreas – The pancreas is responsible for producing glucagon and insulin. Both hormones help regulate the concentration of glucose (sugar) in the blood.

Gonads – The male reproductive gonads, or testes, and female reproductive gonads, or ovaries, produce steroids that affect growth and development and also regulate reproductive cycles and behavior. The major categories of gonadal steroids are androgens, estrogens, and progestins, all of which are found in both men and women, but at different levels.