Hypothyroidism in Severely Iodine-Deficient Rats*

Abstract

The thyroid status of severely iodine-deficient rats was assessed by measurement of the resting metabolic rate (RMR) and liver mitochondrial α-glycerophosphate dehydrogenase (α-GPD). Rats maintained on the iodine-deficient diet for 2 or 3 months showed significantly reduced RMR and α- 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 α-GPD was observed 29 days after the rats were placed on the iodine-deficient diet. However, the decrease in RMR in the same animals was not statistically significant. These results suggest that measurement of liver α- 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.