Wednesday, April 13, 2011

Introduction to Endocrinology: The World of Hormones

Historically, age-related male hormone changes have not been considered problematic, because fertility in men persists until an advanced age. In contrast, women undergo ovarian function failure and require multiple hormone replacements. More careful evaluation in males, however, shows progressive age-related changes, including:

• Decreased muscle mass and strength.
• Decreased vigor or low energy
• Decreased libido
• Insomnia
• Nervousness and depression
• Hair loss

These changes usually begin in a man’s forties and fifties and point towards hormone imbalances and deficiencies which may be considered the male equivalent of menopause, which is called andropause.

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Since all the negative health-related issues are linked with the hormonal balance, we will investigate the source in more details to find out, what hormones are, and why they are so important.

What is a hormone?

A hormone (from Greek ''ρμή'' - "impetus") is a chemical released by one or more cells that affects cells in other parts of the organism. Only a small amount of hormone is required to alter cell metabolism. It is essentially a chemical messenger that transports a signal from one cell to another.

 All multicellular organisms produce hormones; plant hormones are also called phytohormones. Hormones in animals are often transported in the blood. Cells respond to a hormone when they express a specific receptor for that hormone. The hormone binds to the receptor protein, resulting in the activation of a signal transduction mechanism that ultimately leads to cell type-specific responses.

Where do hormones come from and what do they do?

Hormones are secreted (usually into the bloodstream) by a collection of glands inside the body referred to as the "endocrine system." (A "gland" is a group of cells that produces and secretes chemicals into the body.) The major glands that make up the endocrine system include the hypothalamus, the pituitary gland, the thyroid and parathyroids, the adrenals, the pineal body, and the ovaries and testes (the "gonads").

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Hormones can also be produced synthetically in a laboratory setting and are prescribed by doctors to treat disease or hormone deficiencies. For example, if a person has had their thyroid gland removed, a doctor may prescribe synthetic thyroid hormones to replace those that the person's body can no longer produce.

Over fifty different hormones have been identified in the bodies of humans, and more are still being discovered. Hormones influence and regulate practically every cell, tissue, organ, and function of our bodies, including growth, development, metabolism, maintenance/balance of our internal environment ("homeostasis"), and sexual and reproductive function.

How do hormones cause their effects?

Most hormones circulate via the blood, thus coming into contact with all kinds of cells throughout the body. However, a given hormone usually affects only a limited number of cells, which are called "target cells" for that hormone. A target cell responds to a hormone because it bears "receptors" for that hormone. Hormones, like all molecules, have a specific molecular shape, and thus will fit into certain receptors but not others.

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When it binds to the receptor site of a target cell, a molecule might act as an "agonist" or as an "antagonist" (or some combination of the two).

"Agonists" are molecules that bind to the receptor site of a target cell and produce biological effects as a result. For example, when the hormone "testosterone" is secreted into the blood and binds to a target cell's receptor site, biological effects from that binding will result in producing a specific physical change (such as the stimulation of a hair follicle to produce a whisker on the chin). Testosterone is an example of an agonist in this case.

Bear in mind that in many cases, more than one distinct hormone can bind to the same receptor. For a given receptor, different agonists can have dramatically different potencies.

"Antagonists" are molecules that bind to the receptor site of a target cell while at the same time failing to trigger the biochemical results of the agonist. Antagonist molecules may compete with an agonist for receptor sites, thus preventing or blocking the binding of an agonist. For this reason, antagonist compounds are often used as drugs.

An example of an antagonist is the drug Tamoxifen, which serves as an estrogen-receptor antagonist in breast tissue (it is sometimes called an "anti-estrogen" in medical literature). Tamoxifen is used in the treatment of breast cancer. It binds to estrogen target cell receptors in the breasts and blocks the ability of estrogen to produce its biologic effects--one of which is the feeding of the cancer itself.

Interestingly, while Tamoxifen acts as an estrogen antagonist in breast tissue, it also acts as an estrogen agonist in the bones. That is, Tamoxifen produces a mixture of antagonist/agonist reactions, blocking the actions of estrogen in the breasts, while also producing the (positive) biologic effects of estrogen in the bones.

How are hormones regulated in the body?

The production of hormones in the body is almost always regulated by a delicate set of feedback relationships, or "feedback loops." Most (but not all) hormone secretion is governed by "negative" feedback loops, wherein the amount of a substance in a system regulates its own concentration. When concentration of a hormone rises to above desired levels, a series of steps is taken within the system to cause the concentration to fall. Conversely, steps are taken to increase concentration when the level is too low.

Imagine a home-heating system as an example of a simple negative feedback loop. When the temperature of a room rises above the set point of a thermostat, the thermostat is then triggered and shuts off the furnace (the heat feeds back negatively on the source of heat). When temperature drops back below the set point, negative feedback is gone, and the furnace turns back on to produce more heat.

The above example of a feedback system is quite simplified; the body relies on complex positive and negative feedback systems, often involving multiple different hormones, steps, and tissues to regulate bodily functions.

In order to function, the body needs healthy endocrine glands that work correctly, a properly functioning blood supply to move hormones through the body to their target points, receptor sites on the target cells for the hormones to do their work, and a feedback system for controlling how and when hormones are produced and used. Any disruption in that system can cause problems that may require medical intervention.

Categories of Hormones

There are two types of hormones known as steroids and peptides. In general, steroids are sex hormones related to sexual maturation and fertility. Steroids are made from cholesterol either by the placenta when we're in the womb, or by our adrenal gland or gonads (testes or ovaries) after birth. Cortisol, an example of a steroid hormone, breaks down damaged tissue so it can be replaced. Steroids determine physical development from puberty on to old age, as well as fertility cycles. If we are not synthesizing the correct steroidal hormones, we can sometimes supplement them pharmaceutically as with estrogen and progesterone.

Peptides regulate other functions such as sleep and sugar concentration. They are made from long strings of amino acids, so sometimes they are referred to as "protein" hormones. Growth hormone, for example, helps us burn fat and build up muscles. Another peptide hormone, insulin, starts the process to convert sugar into cellular energy.

Sources and Additional Information:

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