http://www.nctimes.com/articles/2006/04/09/science/19_38_544_8_06.txt

Thyroid the body's thermostat

By: BRADLEY J. FIKES - Staff Writer 

The thyroid is a butterfly-shaped organ wrapped around your windpipe, often likened to the body's thermostat.

Thyroxine, a hormone secreted by the thyroid, is the chemical trigger that promotes maturation in the young and controls how fast the body's metabolism runs. To make thyroxine, the thyroid needs iodine. So iodine, easily obtained from iodized salt, is an essential nutrient.

The thyroid is an ancient organ that appeared early in the evolution of animals. It regulates growth and metabolism in mammals, reptiles, birds, amphibians and fish. In amphibians, the thyroid controls metamorphosis from the tadpole stage to adult form.

The axolotl, a salamander, evolved in a low-iodine environment in lakes near Mexico City. It remains in its juvenile aquatic form all its life, but can be induced to metamorphosize by supplying the axolotl with iodine or thyroxine.

In humans, too little thyroxine stunts the growth of children and can cause retardation. A lack of thyroxine in adults causes a slow heart beat, fatigue, a tendency to put on weight, and a sense of coldness. Too much, and the person gets a rapid heart rate, loses weight and feels hot.

A thermostat doesn't act on its own; it needs to be "set" by some outside agent.

The first control over the thyroid is the pituitary gland, a pea-sized organ at the base of the brain that makes a hormone that stimulates the thyroid, logically called thyroid stimulating hormone. When TSH gets to the thyroid, it signals the thyroid to make thyroxine.

The second is the hypothalamus, a structure in the brain that makes two hormones that controls the production of TSH. They are thyroid releasing hormone and somatostatin. Thyroid releasing hormone stimulates the pituitary to make TSH, and somatostatin inhibits production of TSH. The hypothalamus detects the level of thyroxine in the blood and produces these hormones as needed to keep the level from going too high or too low.

Together, this mechanism is called the "hypothalmic-pituitary-thyroid axis," and it usually functions well. But in about 5 percent of people, the balance is disturbed, causing hypo- or hyperthyroidism. Other problems include nodules, or growths, on the thyroid, and thyroid cancer.

Fortunately, nearly all thyroid problems are easily treatable.

Hypothyroidism is treated with artificial thyroxine, taken once a day in pill form.

Hyperthyroidism is more complicated to treat. Medication can lower the level of thyroxine in some cases. But when medication doesn't work, surgical removal of the gland or irradiation may be necessary. To irradiate the thyroid, the patient is given radioactive iodine, which accumulates in the gland, partially destroying it. In those cases, thyroid function may fall below normal, requiring the patient to take artificial thyroxine.

Thyroid cancer patients generally have a good prognosis, with about a 95 percent survival rate after 20 years, according to an Oct. 11, 2002, study in the British journal, the Lancet.

http://www.nctimes.com/articles/2006/04/09/science/19_38_544_8_06.txt

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