Small Animal Clinical Services

Radioactive Iodine Treatment of Hyperthyroid Cats


Hyperthyroid Cats

I-131 Treatment for Hyperthyroidism in Cats

  • Approximate cost (total bill which includes examination fee, I-131, some limited diagnostic evaluation if needed, and hospitalization): $1300-$1500; a cardiology evaluation if needed could add $300 to this estimate.
  • Length of time in hospital: 6 to 8 days. The campus Radiation Safety Office, not the clinician, authorizes release of the cats)
  • Route of I-131 administration: single subcutaneous injection
  • Side effects: none expected, though, as with other treatments for hyperthyroidism, decreases in renal function (please see below) may be observed following I-131 therapy; hypothyroidism is uncommonly observed.
  • Methimazole: We would like to have Methimazole discontinued 2 weeks prior to treatment with I-131. 

Laboratory evaluation

Results of current laboratory tests, including a minimum of BUN, creatinine, urine specific gravity and total T4 during a euthyroid state (or nearly normal T4 ) induced by methimazole treatment, should be available in an otherwise healthy cat. If a cat is showing signs of illness a CBC, chemistry panel, urinalysis, and possibly additional diagnostics may be required prior to injection. Laboratory test results can be sent with the client, or if necessary blood drawn for testing at WSU. Diagnostic testing, including laboratory tests, may be repeated at WSU at the discretion of the admitting/supervising clinician.

We often ask for faxes (509-335-3330) or an email of recent laboratory results- especially the CBC, chemistries and urinalysis- so that any concerns we have may be addressed prior to the trip to Pullman.

Hyperthyroidism and chronic renal failure

Cats with both hyperthyroidism and chronic renal failure may have an increase in kidney waste products (azotemia) after treatment for hyperthyroidism.

For the recently diagnosed hyperthyroid cat, the trial therapy with methimazole is strongly encouraged so that the effect of treatment (normal thyroid hormone concentration) on kidney function can be evaluated. It is generally felt that if serum BUN and creatinine concentrations remain unchanged and urine specific gravity is normal with normalization of T4 after methimazole treatment, then definitive therapy with surgery or I-131 is unlikely to be associated with worsening of kidney function post-treatment.

*Development of azotemia following implementation of methimazole treatment is not an absolute contraindication to I-131 treatment, but does create management concerns that may impact the decision to treat with I-131.

Discharge from the hospital

Length of time in hospital: 6-8 days average (shortest has been 5 days, longest 14 days; the campus Radiation Safety Office, not the clinician, authorizes release of the cats). Discharge instructions will include guidelines on special handling of urine and feces, and maintaining the cat at a safe distance from small children, or pregnant women for two weeks post release.  For further information or to arrange referral, please contact the Internal Medicine Technician, Denise Waiting at 509-335-0711.

Washington State University