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Response of Hyperthyriod, Euthyroid and Hypothyroid Steers to Exogenous Thyroxine^{1, 2,}

University of Wyoming, Laramie 82070⁴

Abstract

One hundred and eight Hereford and Angus steer calves, representing three thyroid classification levels and three thyroxine implant levels within each thyroid classification, were divided into two winter feeding groups. Individual weights and blood samples were collected at periodic intervals. The blood samples were analyzed to determine changes in thyroid activity. The steers fed the grain-roughage gained significantly (P<.01) more than the steers fed only roughage. Significant (P<.01) differences in thyroid status were noted at the beginning of the experimental period and at 4 and 21 weeks post-implant. At 4, 10, 14, 19 and 21 weeks post-implant, the Angus steers exhibited a significantly (P<.01) greater degree of thyroid activity when compared with the Hereford steers. At 2 weeks post-implant, there was a significant (P<.05) interaction between breed and thyroid activity in that the hypothyroid Hereford steers exhibited a greater thyroid activity than the euthyroid and hyperthyroid Hereford steers, respectively. During this same period, the hyperthyroid Angus steers exhibited a greater thyroid activity when compared with the euthyroid and hypothyroid Angus steers, respectively. The thyroid activity of Angus calves at all thyroid classification levels was significantly (P<.01) greater when compared with all thyroid classification levels of the Hereford steers during the same period.

¹ Data from a thesis submitted by the senior author in partial fulfillment of the requirements for the Ph.D. degree, University of Wyoming.

² The authors wish to acknowledge the cooperation of Abbott Laboratories, North Chicago, Illinois, for furnishing the resin-embedded sponges and I-131 buffer material.

³ Present address: University of Nebraska, Scotts Bluff Station, Mitchell.

⁴ Approved by the Director of the Wyoming Agricultural Experiment Station as Journal Article JA 426.