Veterinarians are seeing a lot of hyperthyroid cats, and it is one of the most frequently diagnosed disorders in senior cats. Although only occasionally malignant at the onset, normal thyroid tissue becomes hyperplastic over time, and according to Dr. Mark Peterson, many of the patients will have progressed from hyperplasia to tumor by the time they are diagnosed.1 The thyroid disease will continue to grow and progress, so treatment is indicated.
As we navigate these waters with our clients, it is very helpful to be able to answer their questions about which treatment is associated with the longest survival time.
There are currently four treatment modalities: medical management with oral medications, dietary management with iodine restricted diet, surgical thyroidectomy and radioactive iodine treatment. Medical management using oral pharmaceuticals and dietary management are both reversible treatments that do not provide a permanent “cure” because signs will recur with discontinuation. Obviously, thyroidectomy and radioactive iodine are more permanent solutions since they involve removal/destruction of the thyroid gland(s).
Medical management with methimazole was preferred by 66% of the veterinarians, while 28% favored surgical thyroidectomy in a retrospective study in the UK.2 We have many factors that influence which of the modalities is best for each individual patient, including the ability of the owner to medicate the patient long term.
Other studies have looked at survival following radioactive iodine,3 but in clinical practice (especially those that do not have this modality readily available), it is good to know about survivability of cats treated with the most common choices; oral methimazole versus surgical treatment.
A retrospective study published in Macedonian Veterinary Review in March compared these two treatments.4 They found that treatment with methimazole and with surgery produced comparable longevity with a median survival of almost two years in cats without comorbidities. We do know that because hyperthyroidism is a disease often found in geriatric cats, there is a greater risk that other diseases will be present.
This study looked at these factors as well. Thankfully, no significant impact on survival was found in cats that have both thyroid disease and renal disease, or thyroid disease with a concurrent heart murmur. They did find that the age of the cat at the time of diagnosis decreased longevity regardless of treatment.
Because the surgical intervention and medical management were found, in this study, to have no significant differences in survival time, veterinarians can feel confident in allowing the other factors (such as owner compliance and/or budget concerns) be their guide as they select individual treatment plans. It is important to note that although the cost of surgery is more of an investment on the front end, with a median survival of 23-25 months, one must consider the monthly cost of long-term medication.
Each case is individual and the owner’s input must be sought. However, it is helpful to be able to explain that longevity may not be a factor in their choice. +
1. Peterson. M. E., (2020, September). Hyperthyroidism in Cats: Considering the Impact of Treatment Modality on Quality of Life for Cats and Their Owners. Veterinary Clinics of North America: Small Animal Practice, Volume 50, Issue 5, Pages 1065-1084 https://doi.org/10.1016/j.cvsm.2020.06.004
2. Forrester, S., Towell, T., Shenoy, K. (2012). Nutritional management of feline hyperthyroidism. Conference Proceedings of the Veterinary Medical Association, 130th Annual Meeting, Iowa: p.87-91. https://www.avmi.net/information/hyperthyroid-hints/nutritional-management-of-feline-hyperthyroidism/
3. Slater, M., Geller, S., Rogers, K. (2008, June 28). Long-term health and predictors of survival for hyperthyroid cats treated with iodine 131. J Vet Intern Med. 15(1): 47-51. https://onlinelibrary.wiley.com/doi/10.1111/j.1939-1676.2001.tb02298.x
4. Mata, F. and Bhuller, R. (2022, March 28). Hyperthyroidism in the Domestic Cat (Felis Catus): Informed Treatment Choice Based on Survival Analysis. Macedonian Veterinary Review, vol.45, no.1, pp.71-78. https://doi.org/10.2478/macvetrev-2022-0015