Topic Overview

The thyroid gland uses iodine from food to make two thyroid hormones: triiodothyronine (T3) and thyroxine (T4). It also stores these thyroid hormones and releases them as they are needed. The hypothalamus and the pituitary gland, which are located in the brain, help control the thyroid gland. The hypothalamus releases thyrotropin-releasing hormone (TRH), which stimulates the pituitary gland to release thyroid-stimulating hormone (TSH). When the hypothalamus and pituitary are working normally, they sense when:

• Thyroid hormone levels are low, so they secrete more TRH and TSH, which stimulates the thyroid to make more hormones.
• Thyroid hormone levels are too high, so they secrete less TRH and TSH, which reduces hormone production by the thyroid.

Disease or tumors of the pituitary gland can affect this process.

See a picture of thyroid hormone production.

Thyroid hormones affect every cell and all the organs of the body. They:

• Regulate the rate at which calories are burned, affecting weight loss or weight gain.
• Can slow down or speed up the heartbeat.
• Can raise or lower body temperature.
• Influence the rate at which food moves through the digestive tract.
• Control the way muscles contract.
• Control the rate at which dying cells are replaced.