A molecular model of thyroxine (T4), a thyroid hormone produced in the thyroid gland. Thyroxine (T4) along with triiodothyronine (T3), increases metabolism and affects the growth and development of cells in the body. Thyroid hormones cannot be synthesized when there is a deficiency of dietary iodine. As a result, low levels of thyroid hormones increase production of thyroid-stimulating hormone which enlarges the thyroid, resulting in a condition called goiter. Atoms are colored dark gray (carbon), light grey (hydrogen), blue (nitrogen), purple (iodine) and red (oxygen).