THE TOTAL AND DIFFERENTIAL CONCENTRATION OF RADIO-IODINE IN THYROID TISSUE AND ITS VARIATION WITH TIME, AS STUDIED BY AUTO-RADIOGRAPHY

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Abstract
SUMMARY: 1. Autoradiographs were made from thin thyroid sections of rats injected intraperitoneally with 20μc. 131I and killed at periods varying from 15 min to 16 days. 2. The concentrations of 131I in the thyroid sections were first deduced from G.M. counter measurements and then correlated with the average grain densities of their autoradiographs. From the observed grain densities in the colloid and cellular spaces of the thyroid follicles, the differential concentration of 131I in these sites was determined. 3. Each mm3 of thyroid tissue was found to fix about 0·1μc. in 15 min, 0·25μc. in 3 hr and 0·8μc. in 24 hr, out of the injected dose of 20μc. of 131I. The biological half-life of 131I in the thyroid was found to be 5·4 days. 4. From the knowledge of the total and differential concentrations of 131I, the partial volume of the colloid was calculated to be 66% of the whole thyroid volume, the rest being occupied by the cellular spaces. The latter was found to retain about half of the total 131I of the thyroid up to 15 min after the injection. But after 3 hr most of the 131I appeared in the colloid.