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Comparison of the Thyroid Release of I-131 by Hereford and Brahman Cattle Maintained under Identical Environmental Conditions¹

Florida Agricultural Experiment Station, Gainesville³

Abstract

Twenty-four non-pregnant Hereford and Brahman heifers were injected intravenously with 5 microcuries of 1-131. In vivo monitoring of 1-131 in the thyroid glands of these animals by means of two scintillation probes, which simultaneously fed impulses into one ratemeter, was done for 406 hours after injection. The mean value for time needed for maximum uptake of 1-131 (U) was 70 hours after injection for the Herefords and 94 hours for the Brahmans. The thyroid glands of the Herefords retained a significantly greater percentage of injected 1-131 (K₄) at comparable time intervals than did the Brahman heifers. A reduction of 50% in dietary protein intake of the experimental animals of both species reduced the amount of 1-131 retained by the thyroid. The release rates of 1-131 from the thyroid as measured by the slope of the curves for 1-131 release were much the same for both species and diets. The lower and slower uptake of iodine by the thyroids of the Brahman cattle indicates a less active thyroid in these animals, which may be a factor in their being more heat tolerant than Herefords.

¹ Florida Agricultural Experiment Station Journal Series No.

² Present address: Poultry Science Department, Auburn University, Auburn, Alabama.

³ Department of Animal Science.