Effect of Iodine Deficiency on Iodine-Containing Compounds of Rat Tissues

The distribution and quantitation of endogenously labeled iodine-containing compounds in normal and iodine-deficient rats were determined under steady state conditions. An isotope equilibrium method based on chronic feeding of 125I-labeled diet of known specific activity containing 0.06 μg 127I/g (experimental) and 0.22 fig 127I/g (control) was employed. Iodine deficiency was associated with the usually described rise in thyroid weight and MIT/DIT and T3/T4 ratios, even though there was a significant absolute decrease in all iodine compounds. Despite the iodine deficiency state the absolute concentration of T4 exceeded that of T3. Quantities found per g thyroid were 5.3 μg DIT, 5.7 μg MIT, 2.6 μg T4 and 1.7 μg T3. The T3/T4 ratio in plasma and tissues also rose, but for different reasons, as there was an absolute rise in T3 in plasma and most tissues and an absolute fall in T4 in plasma and all tissues. Despite these changes the absolute concentration of T4 still exceeded that of T3 in plasma and most tissues. The quantities of T4 found per g tissue ranged from 0.75 μg in brain to 8.5 mμg in liver. Ts per g ranged from 0.5 mμg in epididymal fat to 9.3 mμg in kidney. Fecal hormone excretion was decreased. The data suggest that the animal adapts to iodine deficiency with an increased level of plasma and tissue T3, but that it is unable to completely suppress T4 synthesis.(Endocrinology79: 309, 1966)