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Effect of Scurvy on Deiodination of ¹³¹I-labeled Thyroxine in Guinea Pigs

Biochemistry Research Laboratory, Veterans Administration Hospital, and the Department of Biochemistry, The Johns Hopkins University, Baltimore, Maryland

The effect of scurvy on metabolism of thyroxine (T₄) and triiodothyronine (T₃) was investigated. Experimental scurvy was produced in guinea pigs by feeding a diet low in vitamin C for 3 or 4 weeks. Control animals received the same diet and ascorbic acid injections. Deiodination of both thyroid hormones was greatly increased in scorbutic guinea pigs. This was shown by an increasing excretion of ¹³¹I in the urine and a decreasing retention of protein bound ¹³¹I in the plasma, liver and kidney after a single injection of ¹³¹I-labeled T₄. Similar effects were observed after ¹³¹I-labeled T₃ injection. Administration of nonradioactive T₄ or cortisone to scorbutic animals failed to show any significant changes in the metabolism of ¹³¹I-labeled T₄. Normal deiodinating activity was completely restored by daily injection of 15 mg of ascorbic acid for 7 days. Scurvy had no effect on the thyroid uptake of ¹³¹I in vivo, but increased the amount of ¹³¹I-labeled T₃ to the erythrocyte in vitro. Both plasma protein-bound iodine and thyroxine levels were significantly decreased in scorbutic guinea pigs. These low values are probably due to a decrease of plasma protein binding capacity during scurvy.