Various aspects of research in thyroid function and iodine metabolism are under investigation as follows: 1. Purification of thyroid peroxidase and determination of its chemical and physical characteristics. 2. Determination of the role of thyroid peroxidase in iodination and in thyroid hormone biosynthesis. 3. Studies of the mechanism of thyroid peroxidase-catalyzed coupling. 4. Elucidation of the mechanism of action of the clinically used antithyroid drugs. 5. Studies of the effects of severe iodine deficiency in rats. 6. Measurement of serum T4 and T3 levels in cold blooded animals.