Roger's "Practica," written about 1170, prescribed ashes of sponge and seaweed for goiter;1 yet knowledge of the therapeutic value of iodine products in treatment of this disease and of the metabolism of iodine within the body has been developed only within the present century. Clinical and experimental evidence points to the fact that one of the primary functions of the thyroid gland concerns the metabolism of iodine. It is essential, therefore, to acquire as great an insight as possible into the biochemistry of iodine in normal and in goitrous persons. Investigations of the level of iodine in the blood have yielded information of value. The present communication deals with the development of knowledge of the concentration of iodine in the blood together with an analysis of a series of values for blood iodine in 1,078 consecutive patients with clinical hyperthyroidism and in 745 persons without evidence of thyrotoxicosis.