Hypothyroidism in severely iodine-deficient rats.

The thyroid status of severely iodine-deficient rats was assessed by measurement of the resting metabolic rate (RMR) and liver mitochondrial alpha-glycerophosphate dehydrogenase (alpha-GPD). Rats maintained on the iodine-deficient diet for 2 or 3 months showed significantly reduced RMR and alpha-GPD, compared to rats on the same diet supplemented with KI in the drinking water. They also displayed markedly reduced serum T4 levels, slightly reduced serum T3 levels, and highly elevated serum TSH levels. A significant decrease in liver alpha-GPD was observed 29 days after the rats were placed in iodine-deficient diet. However, the decrease in RMR in the same animals was not statistically significant. These results suggest that measurement of liver alpha-GPD may be a more sensitive index of impending hypothyroidism than measurement of O2 consumption. The present study demonstrates that a hypothyroid state can be induced in rats exposed to a severely iodine-deficient diet. In severe iodine deficiency, the compensatory mechanisms of increased TSH stimulation and preferential T3 secretion from the thyroid are insufficient to prevent a fall in serum T3. The hypothyroid state results from the inability to maintain a normal serum T3 level and possibly also from the very low levels of serum T4.