Implications for practice
Previously we were only concerned about iatrogenic hypothyroidism in treated hyperthyroid cats when this was clinically overt.
Usually we made the diagnosis when clinical signs, such as lethargy and heat seeking, were present and concerning to the owner.
The research carried out over the last few years suggests that iatrogenic hypothyroidism is not as innocuous as we thought,
at least in cats that have CRD. In this scenario, hypothyroidism contributes to shorter survival. Making them euthyroid improves
their azotemia and hopefully lengthens their survival time. As a result, if you have an azotemic hyperthyroid cat that is
being treated medically, adjust the drug dosage to prevent hypothyroidism.
As it is, iatrogenic hypothyroidism is much less common in medically treated cats. If treatment was via bilateral thyroidectomy
or radioactive iodine treatment, the chances are good (probably > 80 percent) that the cat does have iatrogenic hypothyroidism.
We have yet to examine if thyroid supplementation should be considered in these cats, though it does appear tempting. Of course,
in some cases, the reason the cat was treated with surgery or radioactive iodine was to avoid administering oral medication
to the cat, so that would make for an interesting conversation with the owner.
Anthony Carr, Dr. med. vet., DACVIM, is a professor of small animal clinical sciences at the Western College of Veterinary
Medicine in Saskatoon, Canada.
1. 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.
2. Graham PA, Refsal KR, Nachreiner RF, et al. Measurement of feline thyrotropin (TSH) using a commercial canine immunoradiometric
assay. J Vet Intern Med 2000;14:342.
3. Williams TL, Elliott J, Syme HM. Association of iatrogenic hypothyroidism with azotemia and reduced survival time in cats
treated for hyperthyroidism. J Vet Intern Med 2010;24(5):1086-1092.
4. Williams TL, Elliot J, Syme H. Restoration of euthyroidism in medically treated hyperthyroid cats with iatrogenic hypothyroidism
(IH) improves renal function, in Proceedings. Am Coll Vet Intern Med 2012;753-754.