Thyroxin thyroid hormone, molecular model. Thyroxin, commonly knows as T4, is a hormone made from tyrosine in the thyroid gland. Thyroid hormones regulate metabolism in virtually every cell in the body. Once in the blood, an enzyme converts T4 to T3 and rT3. All three are used to treat thyroid disorders. Atoms are colour-coded: carbon is black; hydrogen is white; iodine is purple; nitrogen is blue; oxygen is red.