The author of this monograph presents the endocrine function of iodine authoritatively. In addition to the role of free iodine and iodine-containing compounds, the relationship of the various glands of internal secretion to the thyroid gland from the point of view of fundamental biochemistry and physiology is discussed. The book contains some of the most recent contributions to our knowledge of iodine as obtained by using radioactive iodine. The physician will be especially interested in the chapter on clinical problems, in which are included representative case histories and the common problems of diagnosis and therapeusis. Another chapter of special interest to the physician is the one on the relationship between the thyroid and the nervous system. The appendix describes in detail the various technical procedures for the analysis of iodine and iodine compounds in various fluids and tissues. This appendix is a useful reference for the laboratory technician.