Hyperthyroidism Challenges: Treating Cats with Concurrent Renal Compromise

by Dr Dave Nicol and Professor Richard Malik

This week we were referred a case for radio-iodine therapy that always brings up some interesting discussion.

The case was a 15-year-old, female, neutered, domestic-shorthair. The cat was in reasonable body condition and other than a tachycardia and large thyroid goitre (plus moderate periodontal disease) had no outward evidence of co-morbidities.

When referred a case we always ask for some baseline measurements prior to referral:

• Serum biochemistry
• Total T4
• Urine specific gravity (USG )
• Blood pressure

 The cat in question was an excellent example of why. Serum biochemistry demonstrated a mild elevation of both ALT and ALKP, likely attributable to thyrotoxicosis. Urea and creatinine concentrations were within normal limits, though towards the higher end of the reference range. The total T4 level was 140, a relatively high reading. Importantly, the urine specific gravity was 1.025. Finally, the cat’s mean systolic arterial blood pressure was 200mmHg.

Such a case presents a challenge to the practitioner as the normal blood urea/creatinine with low USG can seem paradoxical. However, one of the side-effects of feline thyrotoxicosis is to increase the blood pressure. The knock-on-effect of this is to maintain adequate clearance of urea/creatinine from the kidney due to higher glomerular filtration rate, in spite of the underlying damage. Unfortunately the elevated blood pressure also continues to damage the glomerular component of the nephron, leading to a gradual deterioration in renal condition.

 For the practitioner dealing with these cases it can seem like a choice between a rock and a hard place. Do you treat the condition and risk unmasking serious renal disease? Or do you opt for benign neglect (actively do nothing) and wait for renal and cardiac disease to worsen to the point of no return?

In such cases there is a strong argument for radio-iodine therapy. Unlike surgery, treatment with radioactive iodine leaves behind normal thyroid tissue that (after a few weeks post treatment) will resume production of thyroxine. This normal production is often enough to maintain cats with renal compromise, whilst removing the life-threatening effects on the myocardium and other end-organs. However normal production does take a few weeks to resume following treatment, so a careful management plan is required.

At Double Bay Veterinary Clinic we favour an approach to these cases that includes the following:

1. Careful discussion of risks and potential outcomes with the pet owner.
2. Commencing standard therapy for chronic renal insufficiency while the animal is being treated in hospital – by feeding canned renal support diets, and adding Ipakitine as a phosphate binder.
3. The implementation of a homecare plan following treatment with nuclear medicine that allows a “gentle landing” for the kidneys. To this end we often discharge cats with a supply of thyroxine (50 to 100 micrograms given once daily) which will maintain the circulating blood levels of T4 until endogenous production recovers.

This can take from 4-8 weeks depending on the dose of I¹³¹ administered. Following this time the dose is tapered down allowing the body to take over again.

Mark Petersen speaking at the ECVIM conference in Toulouse a few weeks ago expressed the view that the great majority of cats with IRIS stage II chronic renal disease with concurrent hyperthyroidism should be treated using radio-iodine, with appropriate attention to concurrent treatment of the patient’s renal issues.

For the vast majority of cats where diagnosis is made early, no such additional therapies are required. This highlights the importance of early detection and regular heath examinations for older cats.
However, in those cats (as was the case we treated last week) that have some degree of renal compromise, the fact that nuclear medicine allows an endogenous production of T4 to recover is just one more reason why radioactive iodine therapy is considered the gold standard in therapy for cats suffering from hyperthyroidism.

If you would like to talk to one of our vets about referring a case for radioactive iodine treatment then call us on 93634045. We’d be delighted to hear from you.

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Section Links

Making a Referral to Double Bay Vet Clinic - Information for veterinary surgeons
Feline Hyperthyroidism. A Disease Overview by  R Malik - A detailed overview for veterinary surgeons.
My cat has been diagnosed with an overactive thyroid gland (hyperthyroidism) - Information for pet owners.
Worried about treatment? Read about Milo's experience firsthand from his owner, Rosemary Dunne.

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