What Is the Thyroid Gland?

The thyroid gland is a bowtie-shaped organ in your neck, below the larynx. It secretes hormones that control how fast your heart beats, how quickly you digest food, how much you sweat, the speed at which you burn calories, and many other activities, according to the American Academy of Otolaryngology—Head and Neck Surgery (AAO-HNS).

The thyroid is one of the endocrine glands in the body. Endocrine glands secrete chemical messengers called hormones directly into the bloodstream. Their hormones control functions of other parts of the body. Some of the other endocrine glands are the pancreas, the pituitary, the adrenal glands, the parathyroid glands, the testicles, and the ovaries.

The thyroid makes thyroxine (T4), a hormone that governs metabolism throughout the body. Thyroxine is made from iodine in the thyroid gland. Once in the bloodstream, thyroxine is carried to other organs, such as the liver and kidneys, where it is converted to triiodothyronine (T3), which is a more biologically active form.

The amount of thyroxine made by the thyroid is controlled by the pituitary gland. When the pituitary detects a low level of thyroid hormone, its produces thyrotropin, also called thyroid stimulating hormone (TSH), which causes the thyroid to increase production of thyroxine. Ordinarily, the thyroid produces just enough of the hormone to keep your body running at normal speed. Sometimes, though, it becomes over active or under active.

Thyroid problems are common, the AAO-HNS says. When the thyroid makes too much thyroxine, the condition is called hyperthyroidism; when it makes too little, it is called hypothyroidism. In either case, the thyroid may become enlarged. An enlarged thyroid is called a goiter.

The most common cause of goiter once was a shortage of iodine in the diet. The thyroid needs iodine to make thyroxine. If it doesn't get enough, the gland swells because it's trying to compensate for the lack of iodine. Diet-caused goiter is rare in this country today because table salt has iodine added to it.

Symptoms of hyperthyroidism, according to the American Thyroid Association (ATA), include weight loss, nervousness, irritability, sweating, a racing heart, hand tremors, anxiety, sleep problems, brittle hair, and muscle fatigue, particularly in the upper arms and thighs. The most common type of hyperthyroidism is called Graves' disease. Graves' disease is thought to be an autoimmune condition, in which the body’s own antibodies cause the thyroid to make more hormone than it should.

Symptoms of hypothyroidism, the ATA says, include fatigue; feeling depressed, sluggish or cold; dry skin and hair; constipation; muscle cramps; and weight gain. The two most common causes of hypothyroidism are an autoimmune condition called Hashimoto's thyroiditis and the side effects of treating hyperthyroidism.

An under-active thyroid usually is treated with a daily thyroid hormone medication. An over-active thyroid is usually treated with medication that blocks the thyroid's ability to produce the thyroid hormone.

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