Just Ask the Expert: What is the lifespan of cats with hyperthyroidism?

Dr. Bruyette welcomes endocrinology questions from veterinarians and veterinary technicians.

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Are there any studies, perhaps evaluating age and T4 or creatinine concentrations, that provide statistical comparisons of the lifespan of hyperthyroid cats with and without treatment?

Thomas McCoy, DVM

Havard Avenue Veterinary Clinic

Tulsa, Okla.

This is a good question, but I do not think there is a clear answer. Survival times of hyperthyroid cats after treatment with iodine 131, surgery, or methimazole have been reported. The median survival times reported are 25 (iodine 131 treatment),1 24 (radioiodine treatment),2 and 48 months (iodine 131 treatment),3 with a more recent paper citing 14 months (carbimazole or methimazole treatment alone or in combination with thyroidectomy).4 However, the differences in survival may be related to the cats' age at diagnosis, referral vs. primary care data, and concurrent illness, which may have affected treatment options.

David S. Bruyette, DVM, DACVIM

Since the late 1970s and early 1980s when the first reports of hyperthyroidism and its treatment were published, virtually all cats in which hyperthyroidism has subsequently been diagnosed have been treated. We were all made well aware of the morbidity and mortality associated with the disease, so the profession and pet owners adopted (and appropriately so) a policy of early diagnosis and treatment. We also now know that the disease we diagnose and recognize today is often found incidentally during routine laboratory testing or in cats with mild clinical signs.5 In fact, in cats that are asymptomatic, we discuss the pros and cons of treatment with owners vs. a "watchful waiting approach," which emphasizes owner awareness of clinical signs and periodic veterinary evaluations to monitor for disease progression. Both approaches are acceptable as long as the veterinarian and owner reach the decision fully informed.

No data that I can find prospectively examines the lifespan of untreated hyperthyroid cats compared with the nonhyperthyroid cat population or attempts to compare the survival of untreated hyperthyroid cats with those that undergo treatment. Such studies would be hard to conduct given the safety and efficacy of the various treatment options, the recognized morbidity and mortality that can result from untreated or poorly controlled hyperthyroidism, and the ease of diagnosis and monitoring.

Ultimately, the decision to treat or not treat a cat with hyperthyroidism is left to the veterinarian and pet owner, who make an informed decision by taking into account multiple variables (e.g. age, concurrent illness and medications, severity of clinical signs, suitability or availability of various treatment options) and then deciding which course of action is in the patient's best interest.

David S. Bruyette, DVM, DACVIM

VCA, West Los Angeles Animal Hospital

1818 S. Sepulveda Blvd.

West Los Angeles, CA 90025

Veterinary Diagnostic Investigation and Consultation

26205 Fairside Road

Malibu, CA 90256.

REFERENCES

1. Slater MR, Geller S, Rogers K. Long-term health and predictors of survival for hyperthyroid cats treated with iodine 131. J Vet Intern Med 2001;15(1):47–51.

2. Peterson ME, Becker DV. Radioiodine treatment of 524 cats with hyperthyroidism. J Am Vet Med Assoc 1995;207(11):1422–1428.

3. Milner RJ, Channell CD, Levy JK, et al. Survival times for cats with hyperthyroidism treated with iodine 131, methimazole, or both: 167 cases (1996–2003). J Am Vet Med Assoc 2006;228(4): 559–563.

4. Williams TL, Peak KJ, Brodbelt D, et al. Survival and the development of azotemia after treatment of hyperthyroid cats. J Vet Intern Med 2010;24(4): 863-869.

5. Broussard JD, Peterson ME, Fox PR. Changes in clinical and laboratory findings in cats with hyperthyroidism from 1983 to 1993. J Am Vet Med Assoc 1995;206(3):302–305.