Small, but powerful in its influence on the body, the thyroid gland surrounds the windpipe just beneath the voice box. The thyroid secretes hormones that influence virtually every organ. Thyroid hormone is required for growth and development in children. In adults, its primary job is to regulate the production of metabolic energy. The thyroid governs the "basal metabolic rate," which is a measure of how efficiently the body turns calories into useable heat energy. When thyroid hormone output is low, people are usually tired, cold, and sluggish. They may also be overweight. Thyroid under-activity is known as "hypothyroidism." Without enough thyroid hormone to keep metabolism humming along as it should, we simply do not function very well, physically or mentally.

"Hyperthyroidism" is just the opposite scenario. The thyroid gland is overactive instead of underactive. It secretes too much thyroid hormone. In this case, more of a good thing is definitely not better. Excess thyroid hormone can cause rapid heartbeat. Body temperature is elevated. The hyperthyroid individual may experience extreme weight loss, in spite of a huge appetite, because they burn up calories too fast. Hyperthyroidism can make a person nervous, emotionally unstable, and unable to sleep.

The thyroid gland's hormone-producing activity is controlled by the hypothalamus and pituitary glands in the brain. These glands can sense when the amount of thyroid hormone in the blood is low or high and give feedback to the thyroid accordingly. When thyroid hormone levels drop, the pituitary gland, upon a signal from the hypothalamus, tells the thyroid to make more. The pituitary does this by secreting a hormone of its own called TSH, or "thyroid-stimulating hormone." As the thyroid hormone level rises, the pituitary gland tells the thyroid gland to shut production down. This circle of communication between glands is called a "feedback loop" and it is designed to keep the amount of thyroid hormone circulating in the blood within a fairly narrow range, so we never have too much or too little.(1)

The thyroid gland makes two related hormones: thyroxine, also known as "T4," and triiodothyronine or "T3." Each hormone contains iodine, which is why iodine is required for thyroid function. T4 has four iodine molecules in its chemical structure, while T3 has three. An essential dietary mineral, iodine can be found in seafood, bread, salt, and seaweed. More than 99 percent of all thyroid hormones circulate throughout the bloodstream bound to proteins and do not interact with body cells. The remainder circulating in free form, are the important and active hormones that directly interact with body cells to help regulate metabolism.

Abnormally high levels of thyroid hormone can result in "thyrotoxicosis." Hyperthyroidism is usually, but not always, the cause of thyrotoxicosis. Physicians can order a test called RAIU (radioactive iodine uptake) to differentiate true hyperthyroidism from other causes of thyrotoxicosis. A high RAIU reading indicates hyperthyroidism, with the thyroid overproducing T4, T3, or both.(1)

• Thyrotoxicosis results when tissues are exposed to excessive levels of T4, T3, or both.(2) Like many endocrine disorders, thyrotoxicosis occurs more frequently in women, with an estimated annual incidence of 3 per 1000.