Extrathyroidal Iodide Concentrating Mechanisms

Abstract

The use of radioactive isotopes of iodine has greatly facilitated the study of the extrathyroidal metabolism of this important element at physiological plasma levels. Iodide concentration has been observed in a wide variety of tissues. This process is of considerable intrinsic interest. In addition it is of practical importance in relation to the study of thyroid gland activity with radioiodine and because of the occurrence of these isotopes in radioactive fallout. In vertebrates, iodide concentration is observed in the secretion of certain salivary glands and of the gastric mucosa. In the salivary gland, the ducts are responsible, and in the stomach the mucoid cells of the surface and of the necks of the gastric glands appear to be responsible for this concentration. In the rat, but not in other laboratory animals, an iodide concentrating mechanism has been demonstrated in the epidermal layers of the skin and in the hair follicles. The mammary glands of many animals concentrate iodide in the milk. The site at which this process occurs within the gland but has not been established. The placenta of many animals concentrates iodide and the concentration in fetal blood is higher than in maternal blood in the later stages of pregnancy. There is evidence of a concentration of iodide in the ovarian follicles of mammals. In the laying hen, there is an active concentration of iodide in the yolk of the developing ovocyte in the ovary. This yolk iodide is taken up by the chick thyroid gland during development. There is no convincing evidence for an active tubular reabsorption or secretion of iodide in the glomerular kidney. Active secretion of iodide by the aglomerular kidney of the marine teleost is not established; iodide may be actively secreted by the gill epithelium of these and other fishes. Iodine is found in high concentration in many marine invertebrates, often as iodinated amino acids in the skeletal proteins. There are, however, only a very limited number of dynamic studies reported that clearly establish the existence of an active iodide concentrating mechanism in these animals. Seaweeds have been shown to concentrate iodide by an active process. Little is known about the higher plants with respect to iodide concentration. Many of these iodide concentration mechanisms, like that of the thyroid have been shown to be inhibited by an excess of iodide, by thiocyanate or, more effectively, by per-chlorate and to be dependent upon energy producing metabolic processes. Drugs such as thiouracil, which inhibit the organic binding of iodine, have no effect on the process of iodide concentration. Unlike the thyroid gland, however, these extrathyroidal iodide concentrating mechanisms have not been shown to respond to such procedures as hypophysectomy and thyroxine or thyroid stimulating hormone administration. Nothing is known as to the biological significance of iodide concentration by invertebrates or seaweeds, or by the salivary glands, stomach or skin of mammals. Viewed teleologically, the function of the iodide concentrating mechanism of the placenta, mammary gland, and ovary could be to ensure an adequate supply of iodine to the offspring. The mechanism of the iodide concentrating process at the cellular or molecular level is unknown.