Thyroid hormones are artificially made hormones that make up for a lack of natural hormones produced by the thyroid gland. The thyroid gland, a butterfly-shaped structure in the lower part of the neck, normally produces a hormone called thyroxine. This hormone controls the rate of metabolism -- all the physical and chemical processes that occur in cells to allow growth and maintain body functions. When the thyroid gland does not produce enough thyroxine, body processes slow down. People with underactive thyroid glands feel unusually tired and may gain weight, even though they eat less. They may also have trouble staying warm and may have other symptoms, such as dry skin, dry hair, and a puffy face. By making up for the lack of natural thyroxine and bringing the rate of metabolism back to normal, artificially made thyroid hormone improves these symptoms.

Thyroid hormones also may be...