Are There Different Classes of Hormones?

Hormones may be grouped into one of two general categories: amino acid-based hormones and steroid hormones. The amino acid-based hormones include hormones that are proteins and those hormones that are
derived from the amino acid tyrosine. Examples of protein hormones include insulin, growth hormone (GH), glucagon, and antidiuretic hormone (ADH). Examples of hormones made from the amino acid tyrosine are epinephrine (adrenaline) and thyroid hormone (T3 and T4). Steroid hormones are made from cholesterol and include testosterone, estrogens, cortisol, progesterone, and aldosterone.

What Are Hormones?

There are two ways that one region of our body can communicate with another. The first is by way of nerve impulses and the second is by way of hormones. Hormones are produced by specific organs (glands) in the body including the pituitary gland, parathyroid gland, thyroid gland, hypothalamus, pancreas, stomach, small intestine, adrenal glands, placenta,and gonads (ovaries and testicles) (Table 2.4). Hormones are released into our blood and circulate throughout our body. As they

circulate they can interact with specific cells of a specific tissue and elicit a response within those cells.
 

Only cells that have a specific receptor for a hormone will respond to a circulating hormone. This is an extremely accurate operation. Some hormones may have receptors on cells of only one kind of tissue in our body, while other hormones may have receptors on cells of most tissues in our body. For example, the hormone prolactin stimulates milk production in female breasts. Therefore, the cells associated with the milkproducing mammary glands will have receptors for prolactin, while most other cells in our body will not have prolactin receptors and will not be affected by prolactin. Thyroid hormone and insulin receptors, on the other hand, will be found on the cells of many kinds of tissues in our body.
 

Hormones are produced by specific glands and circulate in the blood to affect the operation of other parts of the body.