FRIDAY, April 8 (HealthDay News) -- The human body's intricate framework of interconnected systems, which work together to maintain health and life, depend on one small, butterfly-shaped gland that weighs less than half an ounce.

The thyroid gland, located in the front of the neck, releases hormones that regulate metabolism, directing the body to break down food into energy and then either use it immediately or store it for later use.

"The thyroid gland is essential to life," said Dr. Peter A. Singer, a professor of endocrinology at the University of Southern California, a past president of the American Thyroid Association and a board member of the American Association of Clinical Endocrinologists. "If you didn't have a thyroid gland, you could not survive."

When disease strikes the thyroid, this directly affects the body's metabolism by altering the amount of hormone produced by the gland.

Too little thyroid hormone results in hypothyroidism, a condition that causes the body to slow down as metabolism lags.

People with hypothyroidism usually feel chronically fatigued, have difficulty concentrating and need to sleep more than normal, according to the U.S. National Institutes of Health. Their body begins to change as its processes slow down, resulting in weight gain, thinning hair, constipation and pain in the muscles and joints.

When too much of the hormone is being produced, hyperthyroidism occurs. In many ways, the symptoms are a mirror image of hypothyroidism: nervousness, irritability, weight loss, difficulty sleeping, rapid heartbeat, hand tremors and diarrhea.

Singer suggested thinking of the body as a car. "If you have a four-cylinder car, you may be going on 6 or 8," he said. "Everything is amped up [with too much thyroid], like you are on adrenaline."

Both too much and too little thyroid hormone most often result from autoimmune disorders that cause the body's immune system to attack the thyroid and interfere with its function, Singer said. The most common cause of hypothyroidism is known as Hashimoto's disease, and the most common cause of hyperthyroidism is called Graves' disease.

"Hypothyroidism is by far more common than hyperthyroidism," said Dr. Alan P. Farwell, an associate professor of medicine and director of the Endocrine Clinics at the Boston Medical Center/Boston University School of Medicine.

Hyperthyroidism affects about 1 percent of the United States population, whereas hypothyroidism affects about 5 percent of the population, according to NIH.

In general, hypothyroidism is more difficult to diagnose because it occurs more often in older people and can be mistaken for the normal slowing down that occurs with aging, Farwell said.

"Hyperthyroidism has some pretty specific symptoms," he said. "There are lots of things that can cause the symptoms of hypothyroidism. It can be harder to tease out what's going on."

Treatment for hypothyroidism is fairly simple, Farwell said. Replacement thyroid hormone is widely available, easy to take and without major side effects for most people.

"It's available in a pill form," he said. "You take one pill, once a day, that's basically replacing the hormone your thyroid normally makes."

Hyperthyroidism can be a bit trickier to treat. Anti-thyroid drugs are available, but they cause side effects for many people. As a result, doctors more often opt to attack the thyroid gland through the use of radioactive iodine.

"In the United States, radioactive iodine is the most effective treatment," Singer said.

The thyroid gland is the only organ in the body that collects iodine, which it uses to make its hormones, Farwell explained. Radioactive iodine damages the thyroid, reducing its ability to make hormone and bringing hormone levels back to normal.

"About 85 percent of people with Graves' disease in the United States eventually are treated with radioactive iodine," he said.

However, most people who are hyperthyroidal and treated with radioactive iodine will eventually develop hypothyroidism because of the damage done to the thyroid gland. "Now the patient is hypothyroidal and needs to be put on a thyroid pill," Farwell said.

Doctors consider this an acceptable trade-off, given that hyperthyroidism can cause more long-term damage to the body and is more difficult to treat with medication, according to NIH.

Radioactive iodine has also been shown to be an effective treatment for thyroid cancer, which most often occurs as a painful lump in the front of the neck.

"Radioactive iodine gives us a magic bullet that goes only to the thyroid, affecting both normal and cancerous cells," Farwell said.

It is such an effective treatment, in fact, that thyroid cancer has a high survival rate. According to NIH estimates, about 44,670 new cases of thyroid cancer were diagnosed in 2010, but only 1,690 people died from it.

"The good news is 85 percent of [people with thyroid cancer] are readily treated," Singer said. "Most thyroid cancers tend to be relatively easily treated."

More information

The U.S. National Institute of Diabetes and Digestive and Kidney Diseases has more about Graves' disease and Hashimoto's disease.

For more on thyroid disease, read about one woman's struggle with Graves' disease.