Antithyroid drugs are of two general types. One type has a small univalent anion of the thiocyanate perchlorate fluoroboride type. This ion inhibits thyroid hormone synthesis by interfering with iodide concentration in the thyroid. It does not appear to affect the iodinating mechanism as such. The other group of antithyroid agents or drugs is typified by thiouracil. These drugs have no effect on the iodide concentrating mechanism, but they inhibit organification. The mechanism of action of these drugs has not been completely worked out, but certain of them appear to act by reducing the oxidised form of iodine before it can iodinate thyroglobulin (Astwood, 1954). On the other hand, there are a few antithyroid drugs of this same general type, such as resorcinol, possessing no reducing activity and possibly acting through formation of a complex with molecular iodine. Any of the antithyroid drugs, of either type, if given in large enough doses for a long period of time will cause goitre, owing to inhibition of thyroid hormone synthesis, with production of hypothyroidism. The anterior lobe of the pituitary then responds by an increased output of TSH, causing the thyroid to enlarge. The effect of drugs that act on the iodide concentrating mechanism can be counteracted by addition of relatively large amounts of iodine to the diet. The antithyroid drugs of the thiouracil type, however, are not antagonised by such means. Besides those of the thiouracil and resorcinol types, certain antithyroid drugs have been found in naturally occurring foods. The most conclusively identified is L -- 5 -- vinyl -- 2 -- thio oxazolidone, which was isolated from rutabaga (Greer, 1950). It is presumed to occur in other members of the Brassica family. There is some evidence that naturally occurring goitrogens may play a role in the development of goitre, particularly in Tasmania and Australia (Clements and Wishart, 1956). There it seems that the goitrogen ingested by dairy animals is itself inactive but is converted in the animal to an active goitrogen, which is then secreted in the milk.