I131 in Milk and Thyroid of Dairy Cattle Following A Single Contamination Event and Prolonged Daily Administration

Abstract

A single I¹³¹ contamination event of forage was simulated by administering I¹³¹ labeled feed to dairy cattle twice daily for 31 days, all feed samples being spiked on day one with 5 µc I¹³¹ per sample. Peak milk concentration in the dairy cattle occurred on the fourth day with about 0.4 per cent of the first day's dose of 10 µc observed per l. of milk. The peak thyroid concentration in the cattle occurred after 1 week and was about 70 per cent of the first day's dose of I¹³¹. In a second study, 5 µc of I¹³¹ were fed daily to three cows starting before parturition and continuing for a period of 6 months of lactation. Apparent equilibrium of I¹³¹ in the thyroid was reached after 2 weeks and approached twice the daily feeding level. A suggestion of increased thyroid uptake occurred following parturition. The I¹³¹ concentration in the colostrum was 2–3 times that in the milk on the seventh day of lactation, after which time a gradual increase was observed. The per cent of the daily dose of I¹³¹ appearing in the milk varied from 0.4 to 1.0 per cent over a 6-month period. Following an increase in the daily stable iodine content of the cow's diet from 5 mg to 2 g per day, milk concentration of I¹³¹ decreased 50 per cent and thyroid concentrations decreased more than 90 per cent.