The use of chemicals to control the hormonal activity of the thyroid gland is now commonplace in medical practice. Although the desirable and the toxic effects of these substances appear to be well known, their mode of action is much less well understood; hence a recent summary by Pitt-Rivers1 will prove useful to those who are interested.
The thyroid possesses the power of concentrating iodide from the circulating blood and then fabricating it into thyroxin, the active portion of the thyroid hormone. The synthesis of thyroxin can be considered to occur in three stages: the conversion of iodide into iodine, which is believed to be an enzymatic oxidation; the iodination of tyrosine to diiodotyrosine, in which the iodine formed in step one is used, and the coupling of two molecules of diiodotyrosine to form one molecule of thyroxin. A substance which has antithyroid activity can presumably interrupt the sequence