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Effects of Orally Administered Goitrogens upon Thyroid Activity and Metabolic Rate in Ruminants¹

Iowa State University, Ames

Abstract

Studies were conducted with cattle and sheep to determine the effect of the goitrogenic compounds, thiouracil and 1-methyl 2-mercapto imadazole (methimazole), on thyroid activity and metabolic rate. Thyroid activity was evaluated on the basis of weight of the thyroid gland and also by two techniques involving radioactive iodine. Metabolic rate was determined on lambs with a carbon dioxide absorption apparatus. Prolonged oral administration to eight steers of thiouracil at levels ranging from 1 to 20 gm. per day had no effect on thyroid activity or carcass quality; feedlot performance was depressed only by the high level. Thiouracil was unpalatable at the higher levels. Methimazole altered to varying degrees thyroid activity in lambs receiving from 4 to 128 mg. per day; it had no observed effect on metabolic rate. Methimazole at levels from 200 to 1200 mg. per day in steers consistently and progressively altered thyroid activity on the basis of both iodine elimination rate from the thyroid and thyroid weight. It increased the weight of the pituitary glands of steers as well as the amount of growth hormone and thyrotropic hormone contained in the gland.

¹ Journal Paper No. J-3726 of the Iowa Agriculture and Home Economics Experiment Station, Ames, Iowa. Project No. 869.

² A portion of the thesis presented by the senior author in partial fulfillment of the requirements for the degree of Doctor of Philosophy, August 1958.

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