The physiological value of the thyroid-parathyroid complex has been the subject of extensive study and voluminous literature without exhaustion of the possibilities, but with the result that we are possessed of knowledge explaining certain phases of abnormality, even if the normal functions be not unexceptionally demonstrated. The accepted alterations of functions are hypothyroidism—inadequate physiology, and hyperthyroidism—excessive activity. Absence or atrophy of the thyroid bodies is usual in hypothyroidism, while enlargements, collectively called goitre, commonly accompany excessive function. Exemplifying the former, cretinism is the result of failure of normal function and development during fetal life while myxedema is the expression of the disappearance of thyroid secretion after it has once been operative; the latter may occur in infancy after nursing has ceased, or at any time that the thyroid may atrophy, during some of the forms of goitre for example. Hyperthyroidism may express itself, with or without visible enlargement of the thyroid body, in nervousness, gastrointestinal disturbances, tachycardia, loss of weight and exophthalmos. The first group, which might be called athyroidism, is often associated with alterations in the bony skeleton in the form of chondrodystrophy or of rickets, while some degree of osseous change is observed with myxedema. Goitre, be it simple or exophthalmic, may be followed by cardiac enlargement or by myocarditis. The character of the thyroid secretion is not known except that it is influenced by the availability of iodine in the diet, but there are some other as yet unexplained features. The normal thyroid fed to cretins or persons suffering with myxedema, has the power to improve the condition apparently by supplying iodine and the other essential elements. Iodine is an important constituent of the gland, being present in combination with protein. Its quality varies indirectly with the amount of colloid and of hyperplasia (Marine). The administration of this element is beneficial in colloid goitre but is harmful in the toxic variety. Even though the administration of thyroid extract may relieve athyroidism, this procedure in normal animals fails to produce typical pictures of hyperthyroidism. Carnivora fed thyroid gland do not show toxic symptoms until excessive amounts are given, whereas herbivorous varieties are much more sensitive to this feeding.[44] Tachycardia, nervousness and exophthalmos are not produced by these experiments, an interesting observation since these signs are not recorded in wild animals, and only vaguely reported by veterinarians. Man is apparently very sensitive to thyroid dysfunction.