Effects of 131I Thyroid Irradiation Damage, Thyroxine, and Thiocyanate on Radioiodine Metabolism in Dairy Cows

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Abstract
This investigation utilized seven pairs of identical-twin cows and two nonpaired cows. Thyroid-damaged cows yielded 53% as much milk containing almost twice as much of a radioiodine dose as their nor- real controls. Other parameters of iodine metabolism as percentages of the control were: ratio of milk radioiodine to plasma nonbound radioiodine, 227; 72-hr thyroid uptake, 17; plasma-bound iodine, 30; and plasma radioiodine half time, 130. Daily subcutaneous injections: to cows with thy- roid damage of 5 mg thyroxine increased milk yield 24%, heart rate 29%, and plas- ma-bound iodine 500%, and resulted in re- ductions of 29% in milk radioiodine, 46% in milk radioiodine to plasma nonbound io- dine ratio, and 40% in plasma half life. These changes were partiMly maintaind by feeding 80 mg thyroxine daily. Feeding 10 g/day sodium thiocyanate in addition to injected thyroxine caused further reduc- tions of 72% in milk radioiodine, 73% in milk radioiodine to plasma nonbound radio- iodine ratio, and 45% in plasma half life. Thiocyanate alone had no effect on milk yield but decreased milk radioiodine 66%, milk radioiodine to plasma nonbonnd ra- dioiodine ratio 77%, plasma half llfe 17% and thyroid uptake 80% in thyroid-dam- aged cows, and 58, 62, 38, and 87%, re- spectively, in normal cows. These results indicate an inverse relationship between circulating thyroxine levels and radioiodine secretion into milk. Thiocyanate produces a greater or over-riding effect on milk ra- dioiodine concentrations.