Iodine functions are essential components of the thyroid hormones thyroxine (T 4) and triiodothyronine (T3), which regulate the rate of energy metabolism. Absorbed iodide is either largely taken up by the thyroid gland for thyroid hormone synthesis or is excreted in urine. Goitrogenic substances in the feed may substantially increase iodine requirements. Goitrogenic substances include both the thiocyanate derived from cyanide in white clover and the glucosinolates found in some Brassica forages such as kale, turnips and rape. Soybean meal and cottonseed meal also have a goitrogenic effect. The thiouracil goitrogens are found in Brassica seeds and inhibit iodination of tyrosine residues in the thyroid gland. These substances impair thyroid iodine uptake. These effects, however, can be overcome by increasing dietary iodine. The action of thiouracil goitrogen is more difficult to reverse with iodine supplementation. Sodium iodide, potassium iodide and ethylenediamine dihydriodide are utilized by animals as sources of iodine. Calcium iodate and pentacalcium orthoperiodate are also of high bioavailability..