Biological Effects of I 131 Continuously Administered to Sheep

Abstract

A four-year study defining the biological effects of chronically administered I131 in sheep is reported. Daily oral administration of carrier-free I131 was employed, the feeding levels ranging from 0.005 to 1800 [mu]c. Diminished thyroid avidity for I131 was evident in all sheep fed 5 [mu]c or more per day, before clinical changes were evidenced. Clinical manifestations of damage in adult ewes fed 240 and 1800 [mu]c/day included lethargy, clumsy motion, constipation, bloating, diminished milk secretion, and dry skin and fleece. A depression in leukocyte count and protein-bound iodine and an increase in nonprotein nitrogen and creatinine values were also observed. The first-year offspring of ewes fed 240 [mu]c/day showed a decrease in hemoglobin and serum calcium and an increase in inorganic blood phosphorus. Growth retardation, variable alopecia, and calcific plaques in the blood vessels were also seen. All ewes fed 240 [mu]c/day conceived during the second breeding season; how-ever, their offspring were stillborn or failed to survive beyond 4 days. All but 2 adults fed 5 [mu]c of I131/day for 3 to 4 years exhibited mild to moderate histopathologic lesions of the thyroid and a significant reduction in gland size. No damage was observed in adult ewes fed 0.15 [mu]c/day for 4 years with a thyroid dose of about 3 rads/week.