Elevation of Serum 3,5,3′-Triiodothyronine and Thyroxine Levels in Rats Fed Remington Diets; Opposing Effects of Nutritional Deficiency and Iodine Deficiency*

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Abstract
In the course of experiments on iodine deficiency induced by Remington diets in rats, we observed that the Remington diet supplied by ICN Nutritional Laboratories, though very deficient in iodine (4 expected on the basis of previous studies with a similarly iodinedeficient Remington diet from another source. In searching for an explanation for this observation, we noted that the ICN Remington diet was nutritionally much more deficient than Remington diets from other suppliers. We also noted that when rats were placed on the iodide-supplemented ICN Remington diet there was a marked increase in serum T3 and T4. In one experiment, rats receiving the ICN Remington diet plus KI in the drinking water for 16 days showed a serum T3 level of 109 ± 16 ng/dl and a serum T4 level of 6.6 μg/dl compared to 52 ± 7.8 and 4.4 ± 0.8, respectively, in control rats on a stock diet. These elevations were not simply the result of increased binding to serum proteins. Serum protein-binding studies by the method of equilibrium dialysis showed a very slight decrease in the percent dialyzable fraction for T3 and T4. However, calculated free T3 levels were significantly elevated (P < 0.001), and increases in free T4, though less striking, were also significant. These elevations were not accompanied by evidence of hyperthyroidism, as judged by measurements of O2 consumption or serum TSH. Although the specific nutritional factors and mechanisms have not yet been defined, our studies demonstrate that nutritional deficiencies in a Remington diet may act to oppose the effects of the iodine deficiency itself. Our observation that the iodide-supplemented ICN Remington diet has a marked serum T3- and T4-elevating effect offers a possible explanation for the blunted thyroidal resposes of rats to the same diet lacking added iodide. Our studies also suggest that alteration of peripheral T4to T3 conversion may not be the only mechanism by which nutritional factors affect serum T3 and T4 levels.