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That a relation exists between the activity of the thyroid gland and the metabolism of iodine is undeniable. The thyroid hormone has a high iodine content.It is now established that iodine is a constant constituent of human blood. There is also a constant daily excretion of iodine in the urine. The level of the blood iodine varies, likewise the amount lost daily in the urine. There is a pregnant significance to these newer facts as applied to the problem of disease of the thyroid.The nature of this relationship between goiter and iodine has been further clarified during the past decade.A part of this has resulted from studies made on the effects of iodinization. More has been learned subsequent to the development and application of more accurate micromethods for the determination of minute amounts of iodine.After extensive investigation it has become apparent that the level of the blood iodine is influenced by changes in thyroid activity. In this our observations confirm those of Sturm, Lunde and other investigators. We shall present at this time a summary of our prin¬ cipal observations, reserving more extensive details for other publications. HISTORICAL BACKGROUNDThe ancient use of burnt sponge in the treatment of goiter was a fortunate empiricism. Following the dis¬ covery of iodine, its presence in sponges was soon demonstrated. As a natural consequence Coindet used iodine in the treatment of goiter in 1820. Many of the problems which he then raised are still unsolved.Since Coindet, iodine and the goiter problem have been inseparable. Subsequent investigation led to Prevost's theory in 1849. This pointed out the relation between iodine deficiency and the incidence of goiter. It was sub¬ stantiated by the chemical studies of Chatin. After the unsuccessful French experiment, in 1860, the sig¬ nificance of iodine received a setback. During the ensuing thirty-five years there arose other theories as to the nature of goiter. It was during this period that the theory of infection gained in credence. This was doubtless due to the contemporary discoveries of Pasteur.Unsuccessful attempts had been made to demonstrate the presence of iodine within the thyroid gland. Baumann accomplished this in 1895. He demonstrated the isolated iodine in tubes, in its characteristic violet vapor form. The significance of this contribution was widely appreciated.Kendall isolated thyroxine in 1919. This crystalline substance possesses certain of the physiologic proper-From the ties of the dried whole thyroid gland. The isolation was repeated by Harington, who determined the correct molecular structure. Working from this he succeeded in synthesizing thyroxine. Thyroxine is 65 per cent iodine.The next advance was in more extensive investiga¬ tion of iodine metabolism. At once arose the necessity of developing methods sufficiently sensitive, and at the same time sufficiently accurate, to determine minute amounts of iodine. The thyroid iodine is readily determined by the older, coarser quantitative methods. This is not true for blood and ur...

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