Thyroxine. Polarised light micrograph of crystals of the thyroid hormone thyroxine. It is also known as T4. It is the most important thyroid hormone, produced in the greatest amounts by the thyroid gland. Thyroxine (with other thyroid hormones) regulates metabolism in the body. In this it is responsible for chemical activity in cells that releases energy from nutrients, or uses energy to create other substances. Thyroxine is essential for normal physical and mental development in children. Secretion of this hormone is controlled by the pituitary gland. Synthetic thyroxine is used as a drug to treat cretinism and goiter. Magnification: x100 at 6x7cm size.