COMPARATIVE BIOLOGICAL EFFECTS OF 125I AND 131I ON THE RAT THYROID

Abstract

ABSTRACT 125I emits no β rays but part of its soft γ radiation is transformed into low energy electrons. Considering the short tissue path length (2-5 μm) of these electrons, it was postulated that its radiation effect on the thyroid might be different from that of 131I (path length 2000 μm). An experiment was designed to compare the biological effect of several dose levels of radiation from 125I on the rat thyroid gland with that from 131I. The effect of varying doses of each isotope on goitrogenesis (methylthiouracil induced) and on thyroid/serum radioiodide concentration (T/S) ratio was measured in animals radiated while on either a normal or a low iodide diet. In relation to 131I, the effect of 125I on goitrogenesis generally was much less than its effect on function. The low iodide diet group given 125I, surprisingly, showed markedly less inhibition of goitrogenesis than the normal diet group, an opposite effect to that attained with 131I. We suggest that the observed differences are due to the gradient of energy absorption in the follicular cell from irradiation from 125I stored in the colloid. The apical cytoplasm would absorb much more energy than the basal cytoplasm and the nucleus, in contrast to the uniform absorption of energy from 131I across the cell. The comparatively lower effect of 125I on goitrogenesis, and the paradoxical reduction in its effect in the low iodide diet groups, may be due to the low energy absorption by the nucleus, especially when, in a taller cell, the nucleus is farther from the colloid.