The thyroid gland produces and secretes thyroid hormones, mostly T4 (inactive) and small amounts of T3 (active). When needed T4 is typically converted in the liver, kidney and other tissues to active T3. The primary action of thyroid hormone is to raise the body’s metabolic rate — that is, to increase the rate of oxygen consumption and energy expenditure. Thyroid hormone is also essential for normal growth and development in children, interacts with other hormones to modulate carbohydrate, fat and protein metabolism, and controls other important biological functions.