Thyroid hormones are made by the thyroid gland. The thyroid gland makes and releases two thyroid hormones: thyroxine (T4) and triiodothyronine (T3). Thyroid hormones affect every cell and all the organs of the body. They:

• Control the rate at which your body burns calories. This affects whether you gain or lose weight.
• Can slow down or speed up your heartbeat.
• Can raise or lower your body temperature.
• Change how fast food moves through your digestive tract.
• Affect muscle strength.
• Control how quickly your body replaces dying cells.

The pituitary gland (located in the brain) makes, stores, and releases thyroid-stimulating hormone (TSH). When TSH is secreted by the pituitary gland, it causes the thyroid gland to release more T3 and T4.