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

The influence of the stable iodine content in the diet on the absorption of radioiodine and its transfer to cow's milk was investigated for cows at di fferent stages of lactation. Three different rates of stable iodine: 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 rad ioiodine was measured for the three different intake rates. The lactation p hase had no significant effect on iodine transfer to milk; therefore, the d ata from the two lactation groups were pooled for further statistical analy ses. The mean transfer coefficient values for oral radioiodine to milk incr eased from 0.020 d l(-1) for the low treatment to 0.024 d l(-1) for the mod erate stable iodine 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 true absorpt ion was complete for all three stable iodine treatments, but rather to diff erential affinities and saturation levels of the thyroid and milk pathways competing for the available iodine. The same behaviour and comparable value s of transfer coefficients (range 0.015-0.020 d l(-1)) were observed for st able iodine. (C) 1999 Elsevier Science Ltd. All rights reserved.