Factors Affecting the Thyroid Uptake of I131 in Dairy Cows

Abstract

Studies of I131 uptake by the thyroid gland of 8 Jersey and Guernsey cows during a period of nearly 18 months have been reported. I131 in the thyroid of the cow reached a maximum of about 18% of the dose at 3 to 5 days after dosing, and after 6 days it was released at a constant rate of about 3% per day. By suitable comparisons it has been shown that thyroid activity reaches a maximum in the first few weeks after parturition but that in later lactation thyroids of dry cows are just as active as those of lactating cows fed the same feed. Iodine in amounts usually included in dairy rations was found to markedly inhibit uptake of I131 by the thyroid, and some evidence that a low-iodine grain ration also inhibited thyroid I131 uptake was observed. Effects which could be attributed to seasonal temperatures were noted, but the average differences found between seasonal measurements were relatively unimportant. Measuring thyroid activity of cows by this method on an uncontrolled survey basis could not be expected to provide reliable comparable data.