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Performance and Iodine Metabolism of Dairy Cattle With Iodine¹³¹ Irradiation Injury

University of Tennessee Agricultural Research Laboratory, Oak Ridge

ABSTRACT

Four mature cows and six growing heifers were given single oral doses of 160–220 µc I¹³¹ kilogram body weight after preliminary measurements of thyroid I¹³¹ uptake, thyroid secretion rate, and metabolism of daily tracer levels of I¹²⁵. Radiation doses to the thyroids were calculated as 12,000 to 27,000 rad. One cow and the identical twins of the six heifers were used as controls. Within ten days, blood plasma I¹²⁵ concentrations of I¹³¹-treated heifers abruptly rose to three times control values, then dropped sharply. The elevated plasma I¹²⁵ levels, most likely from release by damaged thyroids of previously stored I¹²⁵, were accompanied by increased urinary and fecal I¹²⁵ excretions. Thyroid activity six months later was reduced, as shown by average decreases of 64% in plasma protein-bound iodine, 80% in thyroid I¹³¹ uptake, and 56% in thyroid secretion rate. Three cows calved 15, 123, and 136 days after each had received 100 mc of I¹³¹. The 15-day calf was a cretin, with no detectable thyroid activity, the 123-day calf died just after birth, and the 136-day calf was apparently normal. Surviving animals exhibited varying degrees of hypothyroidism six months after treatment. Low milk yields of the laetating cows and growth of the cretin calf were improved by feeding iodinated casein.