Recently, we have shown in the isolated rat thyroids marked effects of excessive exogenous bromide and perchlorate ions on the activity of thyroid peroxidase (TPO), the key enzyme in thyroid hormones (TH) metabolism. Here, we studied in more details, with the aid of several radioanalytical methods, the effects of an enhanced bromide and/or perchlorate intake on various aspects of iodine metabolism and, consequently, on TH metabolism in the rat. Goitrogenic and thyrotoxic effects of excessive bromide and perchlorate ions were followed in adult male rats, maintained on diets with various iodine content, ensuring either sufficient iodine supply or mild to severe iodine deficiency. In rats administered with these xenobiotics, we measured a consistent increase in relative weight of the thyroids with increasing time and concentration of applied bromide, and a sharp reduction of the 24-h uptake of [¹³¹I]-iodide by their thyroids. In these animals, we also determined a steady decline in serum total thyroxine concentration. At the molecular level, we found, unexpectedly, that the influence of exogenous bromide on the TPO enzyme activity in the rat thyroids was not simply inhibitory. It was more complex, biphasic with regard to the extent of bromide intake in the animals. With the use of several radioanalytical methods, including adapted radiometric determination of TPO enzyme activity, we therefore confirmed and quantified the presumed thyrotoxic effects of xenobiotics bromide and perchlorate ions.

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