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The thyroid gland secretes thyroid hormones, which control the speed at which the body's chemical functions proceed (metabolic rate). Thyroid hormones influence the metabolic rate in two ways: by stimulating almost every tissue in the body to produce proteins and by increasing the amount of oxygen that cells use. Thyroid hormones affect many vital body functions: the heart rate, the respiratory rate, the rate at which calories are burned, skin maintenance, growth, heat production, fertility, and digestion.
The two thyroid hormones are T4 (thyroxine) and T3 (triiodothyronine). T4, the major hormone produced by the thyroid gland, has only a slight, if any, effect on speeding up the body's metabolic rate. Instead, T4 is converted into T3, the more active hormone. The conversion of T4 to T3 occurs in the liver and other tissues.
An underactive thyroid produces too little thyroid hormone, resulting in hypothyroidism. ("Hypo" means "under" or "below.") When the amount of hormone released into the bloodstream is below normal, the body uses up energy more slowly, and chemical activity (metabolism) in the cells slows down.
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