Minerals play a variety of essential roles throughout the body. Calcium is the principal mineral constituent of bone and is thus essential for healthy bone structure and function. Calcium also participates fundamentally in blood clotting, nerve conduction, and muscle contraction. Iodine is necessary for the synthesis of the thyroid hormones thyroxine and triiodothyronine. These thyroid hormones regulate the body's metabolic rate. Magnesium is involved in energy metabolism, and is notably important in the heart, skeletal muscles, and nervous system. Copper regulates iron metabolism and activates superoxide dismutase, a powerful endogenous antioxidant. Zinc is important for growth, immune system function, protein synthesis, antioxidant mechanisms, and wound healing. Chromium is required for normal blood sugar and lipid metabolism; it is an integral component of glucose tolerance factor (GTF). Manganese is essential for antioxidant systems in the body, bone growth, fat metabolism, and protein, nucleic acid, and cartilage synthesis. Potassium is involved in normal muscle tone, nerve function, and many enzymes. Selenium is an essential cofactor of glutathione peroxidase, a potent antioxidant.