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Effect of Mild Hyperthyroidism upon Secretion of I¹³¹ in Milk of Dairy Cattle¹

Department of Dairy Husbandry, University of Missouri, Columbia

ABSTRACT

Six lactating dairy cows were administered 100 µd. I¹³¹ intrajugularly. Samples of milk were analyzed daily for five days for the content of I¹³¹ present. During the first 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 five days was 4.8, with a range of 3.4 to 6.0%. The per cent decline in I¹³¹ 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.

¹ Contribution from Missouri Agricultural Experiment Station. Journal Series No. 2345. Approved by the Director.

² Present address: Institute of Experimental Pathology, Jewish Hospital, St. Louis, Missouri.

³ Aided in part by a grant from U. S. Atomic Energy Commission. [Contract AT (11-1)-301].