Prospective randomized comparison of intravenous versus subcutaneous administration of radioiodine for treatment of hyperthyroidism in cats

One hundred twenty cats with hyperthyroidism were treated with radioiodine (¹³¹I); 60 cats were administered radioiodine SC, and 60 cats were administered radioiodine IV. Before treatment, radioactive tracer studies were performed on each cat to determine peak radioactive iodine uptake and effective half-life. These data were used to calculate the dose of radioiodine that would have to be given to each cat to deliver 150 Gy to the thyroid tissue. The 2 groups of cats were similar with regard to age, tracer study results, and radioiodine dose. Mean estimated thyroid mass was larger for cats treated IV, but mean serum thyroxine concentration was higher for cats treated SC. Route of administration did not affect thyroidal uptake of radioiodine. However, radiation exposure of personnel was significantly lower with SC administration than with IV administration, even when IV catheterization was performed. Eighty-five percent of cats treated IV and 84% of cats treated SC were euthyroid 4 years after treatment. Six percent of the cats became hypothyroid after treatment. When compared with IV administration, SC administration of radioiodine appeared to be as effective for treatment of hyperthyroidism, safer to personnel and less stressful to the cats.