Radioactive Iodine Therapy for Feline Hyperthyroidism: Efficacy and Administration Routes

Mooney C.T.

J Small Anim Pract, 1994. 35(6): p.289-294.


The efficacy of  radioactive iodine (131I) administration was studied in a series of 50 hyperthyroid cats. The dose administered to each cat was based on the clinical severity of the thyrotoxicosis, the serum total thyroxine (T4) concentration and the size of the goitre estimated by palpation. The activity ranged from 80 to 200 MBq (mean +/- SD, 143 +/- 24 MBq). The 131I was injected intravenously in 27 cases and subcutaneously in 23 cases. Each cat was hospitalized for 30 days after the injection. Regardless of the route of injection, none of the cats exhibited any side effects after therapy and all tolerated the hospitalization period well. There was a significant (P<0.001) decrease in the serum total T4 concentration (reference range, 10.4 to 42.0 nmol/litre) from a mean +/- SD of 181.3 +/- 111.4 nmol/litre (range, 43.8 to 575.6 nmol/litre) to a mean +/- SD of 19.0 +/- 29.6 nmol/litre (range, 2.0 to 175.7 nmol/litre) 30 days following the injection of the radioisotope. Five cats remained hyperthyroid, although in each case the serum total T4 concentrations had decreased from pre-treatment values. Two of the cats subsequently became euthyroid within three and five months of therapy, respectively, two were lost to adequate follow-up and the remaining cat was successfully retreated four months later. Before treatment, four of these cases had high scores based on the three criteria used for dose estimation. Serum total T4 concentrations below the reference range developed after treatment in many cases, but were often transient. Clinical evidence of hypothyroidism was not apparent in any cat. Recurrence of hyperthyroidism  has not occurred in follow-up periods of up to 32 months. There was no difference in the outcome between the cats injected intravenously or subcutaneously and the latter was considered to be safer and simpler. The administration of an approximated dose of 131I proved to be an effective treatment for hyperthyroidism in 47 (94.0 per cent) of the cats and obviated the need for sophisticated nuclear computer facilities. There may be a lag period in some cases before euthyroidism is achieved and this should be considered before the administration of a second dose. 131I can be administered subcutaneously without untoward effects.