Metabolism of Different Forms of Iodine in Milk Given to Calves by Suckling and Rumen Infusion

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Abstract
Milk labelled with I131 by dosing lactating cows was fed to 25 calves. The percentage of milk I131 in protein-bound form ranged from a normal of 10 up to 90% in anion exchange resin-treated milk. The metabolism of the milk-bound I131 was compared to exchangeable I125 added to the milk just before calves were dosed by nipple pail (suckling), via stomach tube to a nearly empty rumen (infusion), or via stomach tube to a full rumen (drenching). Differences in metabolism of bound and exchangeable I were determined from the ratios of percentage of the dose of I125 to I131 which appeared in blood, urine, and feces at successive 24-hr samplings, and in bile, abomasum, and thyroid 24 to 96 hr after dosing. Only slight differences in metabolism of the 2 forms of I resulted after suckling or drenching. After infusion into the rumen, much of the bound iodine was absorbed as such, then it was excreted into the urine about twice as efficiently as was the exchangeable I source. This resulted in an increase in the proportion of the exchangeable iodine (I125) in plasma, abomasum, andfeces; and about twice as much I125 as I131 was taken up by the thyroid gland. Utilization or excretion of I is markedly affected by the form in which it is present in the blood which, in turn, is affected by the manner of consumption, digestion, and absorption.