Radioactive Iodine as an Indicator in the Study of Thyroid Physiology. ∗

The known facts of thyroid physiology indicate that iodine is selectively taken up by the thyroid gland, and that in some measure that gland's function is regulated by its iodine content. Artificial radioactivity may be induced in a variety of elements by means of neutron bombardment. It seemed that the possibility of using “tagged” (radioactive) iodine as a physiologic indicator was one which demanded investigation.

Ethyl iodide (600-1000 cc) was irradiated in a paraffin-surrounded bottle by immersing in it a neutron source consisting of 110 mg of radium mixed with beryllium in a sealed tube. The radioactive iodine thus obtained was concentrated by a method which has been described elsewhere. 1 This method gave a precipitate of radioactive silver iodide, which was dissolved in a solution of 0.5-1.0 g of sodium thiosulphate, and then diluted to 10-15 cc for intravenous injection. In a series of 48 rabbits, no toxic effects from the acute administration of such quantities were experienced. Aliquot portions of the solution of radioactive iodine used for injection were withheld for measurement of radioactivity.

A Geiger-Müller counter connected to a suitable vacuum tube amplifier and register to record the individual disintegrations of the radioactive atoms was used for detection of the radioactive iodine in various tissues. This apparatus is standardized and has been fully described elsewhere. 2 3 4 Since no gamma rays could be observed from radioactive iodine,¹ determinations had to be made from the beta radiations. The half-period of radioactive iodine is 26 minutes, so that accurate measurements could not be extended beyond about 40 minutes after injection, with the activities available to date.