The peripheral distribution and metabolism of thyroxine in mice.

Within the last decade the availability of several radioactive isotopes, principally I^131, has given great impetus to studies in thyroid physiology. By the use of this radio-element, evidence has accumulated to show that the circulating thyroid hormone is thyroxine, loosely bound to plasma proteins. It was considered of importance to investigate the fate of this hormone in the body. This was done by means of thyroxine labeled with radio-iodine. In the past, this problem has been investigated in normal animals. The present investigation was intended to find out whether or not the fate of thyroxine in the body was influenced by physiological factors such as thyroid extirpation or diet and by pathological conditions such as the presence of mammary tumor. The mo se, a species in which the fate of radiothyroxine had not been previously investigated, was chosen for this work because of two considerations. Radiothyroxine with high specific activity was available only in limited amounts. The size of the mouse - the smallest mammal available - it was thought, would make it possible to increase the concentration of radioactivity to the maximum attainable in any given organ or tissue. Furthermore, strains of tumor-bearing mice were readily obtainable and, therefore, the influence of tumors could be examined in this species.[...