Investigations of the physiological activity of the thyroid gland have shown that the internal secretion of the gland serves many different functions. One or more of a series of symptoms accompany cases of thyroid deficiency. These symptoms are relieved by the administration of the thyroid gland of certain animals, as the sheep, hog, and ox. When this treatment is stopped the symptoms return. It would thus appear that the functions of the internal secretion of the thyroid may be fulfilled by furnishing the body with the constituents of the thyroid gland from another animal.The separation of the various chemical constituents briefly described above suggested the possibility of determining which constituents controlled the various symptoms occurring in cases of thyroid deficiency.The first step in the method of separation of the constituents results in two solutions. One of these contains about 60 per cent. of the total iodine and 9 per cent. of the nitrogen. This is designated “Solution A.” The other, call...