The thyroid gland is located in the front of the neck. It sits below the voice box (larynx). The small, two-inch gland consists of two lobes, one on each side of the windpipe, connected by a small bridge of thyroid tissue called the isthmus.
The thyroid tissue is made up of two types of cells: follicular cells and parafollicular cells. Most of the thyroid tissue consists of the follicular cells, which secrete the iodine-containing thyroid hormones. They consist of thyroxine (T4) and triiodothyronine (T3). The parafollicular cells secrete the hormone calcitonin. In humans, calcitonin has only a minor role in calcium regulation.
The thyroid plays an important role in regulating the body's metabolism. Virtually every tissue in the body is affected or regulated by thyroid hormone. It regulates the brain and nerve development and function, skin, hair, eyes, heart, and intestine function. The thyroid hormones enter into tissues and regulate how those tissues produce or do not produce certain proteins. The thyroid function is controlled by the pituitary, which sits at the base of the brain. The pituitary is controlled by a region in the brain called the hypothalamus.
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