A principal consideration in this series of studies is the relation of the blood iodine level to the basal metabolic rate in various physiologic and pathologic states. Within recent years there have been an abundance of investigations directed toward the understanding of the nature of blood iodine, and many features have thus been revealed. But as yet its essential composition and the exact relation it bears to the thyroid hormone is unknown. Its clinical value, notwithstanding, has become increasingly apparent, particularly for patients with thyroid disease, for whom the basal metabolic rate alone is inadequate as a diagnostic measure. Its theoretic value in disclosing causal relationships in certain diseases may prove of even greater significance. For these reasons, we have made a comparative analysis of the blood iodine, basal metabolic rate and certain other factors in 138 different clinical conditions in a total of 818 persons. Our earlier blood iodine