Effect of Mild Hyperthyroidism upon Secretion of I131 in Milk of Dairy Cattle

Abstract

Six lactating dairy cows were administered 100 uc I131 intrajugularly. Samples of milk were analyzed daily for 5 days for the content of I131 present. During the 1st 24 hr. from 4.9 to 7.2% of the injected dose of I131 appeared in the milk, with a mean of 5.8%. During the five-day period, the total mean percentage of the injected dose increased to 8.2, with a range of 6.9 to 9.2%. The experiment was repeated with the same cows during a period when each cow was receiving exogenous thyroxine equal to 50% above its individual winter thyroxine secretion rate. In spite of the fact that daily milk secretion increased an average of 22%, the mean percentage of the injected dose of I131 appearing in the milk during the first 24 hr. was reduced to 2.9, with a range of 1.9 to 3.9%. The mean total percentage of I131 appearing in the milk during 5 days was 4.8, with a range of 3.4 to 6.0%. The per cent decline in Il31 in the milk followed an exponential course in both experiments. Mild hyperthyroidism thus reduced the I131 secretion in milk 50% the first 24 hr., and 41% during the five-day period. It was pointed out that dairy cattle transfer relatively low amounts of I131 to their milk and that this amount is further reduced by mild hyperthyroidism.