Thyroid Gland: Facts, Function & Diseases
The thyroid gland stretches across the front of the neck, below the voice box.
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The thyroid gland is a small, butterfly-shaped organ in your throat, and it is very important to your health and well-being. The thyroid is part of the endocrine system, which is essentially a collection of glands. Glands produce hormones that regulate mood and various functions in the body. 

"Thyroid hormones impact a host of vital body functions, including heart rate, skin maintenance, growth, temperature regulation, fertility and digestion," said Dr. Jerome M. Hershman, a professor of medicine at the David Geffen School of Medicine at UCLA and author of the thyroid sections of the Merck Manual.

The thyroid gland is 2 inches (5 centimeters) wide and it weighs between 20 and 60 grams (0.7 to 2.1 ounces), according to the U.S. National Library of Medicine. The gland stretches across the front of the neck, below the voice box. Like a butterfly, it has two wings called lobes that stretch around the windpipe. The wings are connected by a small piece called the isthmus.

The thyroid gland, in particular, controls just about every cell in the human body. It secretes hormones calcitonin, T4 (thyroxine, or tetraiodothyronine) and T3 (triiodothyronine) into the bloodstream. The hormones control the rate at which cells and organs turn nutrients into energy and the amount of oxygen cells use. 

"In this way, the thyroid gland is the body's master metabolic control center," said Cindy Samet, a chemistry professor at Dickinson College in Carlisle, Pennsylvania. "Brain, heart and kidney function, as well as body temperature, growth and muscle strength — and much more — are at the mercy of thyroid function."

The thyroid also regulates the brain and nerve function and development, plus the function of the skin, hair, eyes, heart and intestines.

The thyroid works in conjunction with the pituitary gland. The pituitary gland regulates how much hormones the thyroid releases. 

The thyroid gland produces a large amount of T4, but that hormone is not very active. T4 is converted to T3 via an enzyme that removes one of its iodine atoms. "Although T3 is much more potent than T4, there is much evidence to support that people with hypothyroidism feel much better when they receive a combination treatment that includes a small amount of T3 with the commonly prescribed T4," Samet said.

Thyroid conditions affect an estimated 20 million Americans, and as many as 60 percent of people with a thyroid disease don't know they have a problem, according to the American Thyroid Association

"Women are particularly at risk for a thyroid issue," Hershman said. "One in eight women will develop a thyroid disorder during the course of their lives — that's five to eight times the rate in men."

One of these conditions is called Graves’ disease, which was discovered by the Irish doctor Robert James Graves in 1835. Graves' disease is an autoimmune disorder. The immune system attacks the thyroid gland and makes it become overactive. An overactive thyroid is a condition called hyperthyroidism. A bulge on the neck, called a goiter is a common symptom. The patient may also experience an increased heart rate, according to the National Library of Medicine (NLM).

Hypothyroidism is the opposite of Graves' disease. It occurs when the thyroid gland does not produce enough hormones. This can cause body functions to slow down or stop completely. Screening newborns for hypothyroidism is mandatory in the United States, Hershman said. 

Cancer is another disease that can affect the thyroid. A 2017 study published in in the Journal of the American Medical Association (JAMA) has found that between 1975 and 2013 the number of thyroid cancer cases diagnosed each year has more than tripled. Cases of advanced forms of the disease rose by about 3 percent each year while deaths rose by about 1 percent each year. [Related: Thyroid Cancer Rates Triple, and Scientists Look for Cause]

A lump or swelling on your thyroid gland is called a nodule. Nodules can be harmless, but some can cause a production of too much hormone or be cancerous, according to the Endocrine Society.  

"Up to 70 percent of middle-age females and 40 to 50 percent of middle-age males have thyroid nodules, said Dr. Melanie Goldfarb, an endocrine surgeon and director of the Endocrine Tumor Program at Providence Saint John's Health Center in Santa Monica, California, and an assistant professor of surgery at the John Wayne Cancer Institute in Santa Monica. "You can also be born with an extra piece anywhere as high up as the base of your tongue."

In some medical cases, such as cancer, the thyroid is removed. Humans can live without their thyroid if a hormone pill is taken daily.

For the thyroid to remain healthy, it needs iodine to produce hormones, but just a little. "It turns out that one teaspoonful of iodine is enough for a lifetime of thyroid hormone production. But the thyroid gland needs a constant supply of iodine, so we must intake iodine in some form on a daily basis and not all at once," Samet said. Too much iodine can actually make the thyroid produce less hormones. 

The best way to get iodine is through eating healthy foods, such as seafood and dairy products. You can also get it by seasoning your food with iodized salt. Iodine was first added to salt in the United States in the 1920s to combat goiters.

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