The true absorption of 131I, and its transfer to milk in cows given different stable iodine diets

The influence of the stableiodine content in the diet on the absorption of radioiodine and its transfer to cow's milk was investigated for cows at different stages of lactation. Three different rates of stableiodine: a low intake rate of 4 mg d−1, a moderate intake of 35 mg d−1 and a high rate of 75 mg d−1 were fed to two groups of three lactating cows in mid- and late-lactation. The transfer to milk of a single oral administration of radioiodine was measured for the three different intake rates. The lactation phase had no significant effect on iodinetransfer to milk; therefore, the data from the two lactation groups were pooled for further statistical analyses. The mean transfer coefficient values for oral radioiodine to milk increased from 0.020 d l−1 for the low treatment to 0.024 d l−1 for the moderate stableiodine rate. There was a statistically significant decrease in the transfer to milk for the high stable dietary iodine intake rate (mean transfer coefficient=0.018 d l−1) compared with the moderate treatment. These differences were not due to effects on absorption since trueabsorption was complete for all three stableiodine treatments, but rather to differential affinities and saturation levels of the thyroid and milk pathways competing for the available iodine. The same behaviour and comparable values of transfer coefficients (range 0.015–0.020 d l−1) were observed for stableiodine.