Effect of a Limited Iodine Diet on Iodine Uptake by Thyroid Glands in Hyperthyroid Cats

Background: The effect of feeding a limited iodine diet on radioactive iodine uptake in the thyroid glands of hyperthyroid cats is unknown. Objectives: To determine how feeding limited dietary iodine affects radioactive iodine uptake by the thyroid glands of hyperthyroid cats. Animals: Eight geriatric cats with spontaneous hyperthyroidism. Methods: Prospective study of eight client owned hyperthyroid cats fed a commercially available iodine limited diet for 6 months. Clinical signs were evaluated and TT4 and fT4 were measured during consumption of the diet. Uptake of 123 I was determined before and 8- 16 weeks after exclusive consumption of the diet. Results: Clinical signs of hyperthyroidism resolved in all cats, but there was no significant increase in body weight. TT4 and fT4 decreased into the reference range by 8- 16 weeks in all cats. Mean TT4 before consumption of the diet was 9.7 lg/ dL ( SD 5.2) and after consumption of the diet was 3.1 lg/ dL ( SD 0.9). Scintigraphy revealed unilateral uptake of isotope in 5 cats and bilateral uptake in 3 cats. Mean percentage uptake of 123 I by the thyroid gland at 8 hours after isotope administration was 16.2 ( SD 11.8) before diet consumption and 34.6 ( SD 11.7) 8- 16 weeks after exclusive consumption of the diet. The percentage increase was variable between cats ( 38- 639%). Conclusions and clinical importance: Limited iodine diets increase iodine uptake in the autonomous thyroid glands of hyperthyroid cats. Further studies are necessary to determine if consumption of a limited iodine diet changes sensitivity of the thyroid gland to 131 I treatment.