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The thyroid gland is located in the front part of the neck below the larynx (voice box) and just above the clavicle (collarbone). Its shape resembles that of a "bow tie" consisting of two lobes (right and left) that are joined together in the middle by a narrow band of tissue called the isthmus. The thyroid is one of the most important endocrine glands because it regulates the body's metabolism, growth, and development.

Among its many functions, the thyroid gland takes iodine found in the food we eat and combines it with an amino acid called tyrosine to make two hormones called:

• Triiodothyronine (T3)
• Thyroxine (T4)

Most of the thyroid gland consists of follicular cells that produce and store these hormones. The proportion of hormones produced by the follicular cells is approximately 80% (T4) to 20% (T3).

Thyroid hormones perform several important functions including:

• Controlling the rate of metabolism of every cell in the body (conversion of oxygen and calories to energy)
• Maintaining body temperature

The thyroid gland produces hormones in response to stimulation by thyroid stimulating hormone (TSH), which is produced by the pituitary gland. TSH operates through a feedback system with the thyroid gland - when there is a drop of thyroid hormone levels in the blood, TSH stimulates the thyroid gland to produce more thyroid hormones. When the level of thyroid hormones increases, the pituitary gland decreases its production of TSH.

Thyroid hormone production is dependent upon an adequate intake of dietary iodine. The thyroid gland takes up the amount that it needs for hormone production and the kidneys excrete the remainder. In the United States, most people ingest adequate amounts of iodine due to the iodine enrichment of many foods.