Radioactive Iodine Therapy for Hyperthyroid Cats

From 2004 and 2012, 41 hyperthyroid cats were treated with oral radioactive iodine (I-131) at the Adelaide Veterinary Specialist & Referral Centre’s animal medicine department. Since then, we have conducted ongoing follow-up surveys of these cats and are sharing these findings with you to assist your treatment discussions with owners of hyperthyroid cats.

The I-131 therapy treated cats ranged from seven to fifteen years of age (median twelve years). Twenty-seven cats were female and 14 were male (all desexed). All except three cats (one British Shorthair, one Siamese and one Oriental) were domestic “moggie” cats. The pre-treatment total of T4 ranged from 58 to 484 nmol/L (median 152.5 nmol/L).

The median dose of I-131 administered was 205 MBq (165-250). The median hospitalisation time after treatment was eight days (5-15).

Results

Post-treatment T4 values were available for 34 cats and hyperthyroidism was corrected in all cases. Sixteen of 36 cats reassessed at six weeks post-treatment had low T4 values. Six months post-treatment, nine had regained euthyroidism, two had non-thyroidal illness without clinical signs of hypothyroidism and two were not re-evaluated.  Only one relapse was documented, a nine-year-old spayed female Domestic Shorthair cat treated with 250 MBq I-131 that was diagnosed with a functional thyroid carcinoma five years after treatment.

Sixteen cats with follow-up of six months or longer were known to have died since treatment, with a median post-treatment survival time of 34 months (range 4-73 months). Reported causes of death were kidney failure (3), old age (2), cancer (2), stroke (1), failure to recover from anaesthesia (1) and kidney and liver failure (1). No cause of death was given for six cats. One cat “disappeared” a few weeks after treatment (not included in survival analysis).

For 28 cats with sufficient follow-up information available, azotaemia was seen in 15 previously non-azotaemic cats six weeks post-treatment. Eight of these cats had low post-treatment T4 values. Fourteen cats non-azotaemic before treatment progressed to IRIS kidney disease stage 2 and one cat progressed to IRIS kidney disease stage 3, surviving only four months post-treatment.

Four cats were mildly azotaemic (IRIS stage 2 kidney disease) at the time of treatment, and creatinine values remained stable in all cats six weeks post-treatment. One progressed to IRIS stage 3 at six months and survived 17 months post-treatment, with the reported cause of death being “cancer”, one died 36 months post-treatment (“acute renal failure”) and the remaining two had survived 13 and 36 months at the time of follow-up.

Owner satisfaction

To date, we have successfully contacted owners of the 24 cats treated between 2004 and 2012 to determine their long-term satisfaction after I-131 treatment. 22/23 owners who responded to this question reported being satisfied with their cat’s I-131 treatment; one owner was “unsure” and reported having another hyperthyroid cat treated with oral carbimazole that survived longer than their I-131-treated cat, who had died (cause not specified) 18 months after successful treatment.

21/24 owners of cats indicated they would recommend I-131 treatment. Owners who responded negatively included the owner who was “unsure” on satisfaction, one who reported a “changed attitude” in the cat after treatment and the owner of the cat who disappeared shortly after treatment.

Discussion

Our survey findings are consistent with the published literature on radioactive iodine treatment for feline hyperthyroidism and serve to emphasise the following points:

• Radioactive iodine is a highly effective single treatment for feline hyperthyroidism, with high rates of owner satisfaction.
• Average survival for hyperthyroid cats treated with radioactive iodine exceeds two years, with most cats succumbing to unrelated “old-age” diseases.
• Restoration of euthyroidism can unmask renal insufficiency in up to half of cats treated, but in most cases the decline in renal function is mild. It has been recently suggested that low post-treatment T4 values may be associated with an increased risk of developing renal insufficiency. An individualised rather than fixed dose of I-131 is therefore chosen for each patient, to avoid an excessive dose whilst maintaining efficacy. A temporary period of thyroxine supplementation post-treatment may also offset this risk. Cats with pre-existing renal insufficiency can be safely treated, but a treatment trial with reversible anti-thyroid medication before I-131 is recommended to ensure acceptable euthyroid renal status.

The current cost of I-131 radioactive therapy treatment for cats is $1750. A consultation is required before treatment to assess each patient’s suitability for I-131 treatment. After treatment cats are hospitalised in isolation until they emit no more than 1 microsievert/hour of radiation at one metre. Once discharged continued isolation at home for a further two weeks with special handling of litter is required.

Please feel free to contact us with any questions, feedback, or suggestions regarding I-131 treatment in hyperthyroid cats. You can also read more of our specialist veterinary news and treatment stories here.