Biologically available iodine in goitrogenic diets

Eight different sources of low-iodine diet (LID) were tested in mice over 14 years. The available iodine in each diet was measured by isotopic equilibration. Commercially prepared Remington diets contained 6.8 to 69.3 ng available iodine/g, and the results were usually different from shipment to shipment. Some samples produced greatly enlarged thyroids. The Remington diets from two sources were occasionally low in iodine but produced little thyroid enlargement. Between 1977 and 1980 only one shipment of Remington diet was found to contain less than 10 ng available I/g, and it resulted in large goiters. Since 1980 other compositions of LID have been used, but they caused additional abnormalities during breeding or chronic feeding. A low-iodine wheat diet produced goiter in mice more readily than in rats. In the course of testing for unavailable forms of dietary iodine, it was found that only 34.2% of thyroxine iodine was available to the thyroid iodine pool of mice. It is concluded that unidentified nutritional deficiency or dietary contaminants can alter the goitrogenic response to restricted iodine intake. Furthermore, at least one natural form of potential dietary iodine is incompletely available to mice.