Feline Hyperthyroid Clinic FAQ for vets

The treatment involves either the administration of a subcutaneous dose of a radioactive iodine isotope (I131) or an oral capsule. The iodine is uptaken by the hyperactive, hyperplastic or adenomatous thyroid tissue. Once uptaken the I131 releases beta radiation, which travels up to 2 mm destroying all hyperactive tissue but sparing the nearby parathyroid glands. The atrophic normal tissue does not uptake the I131 and is also spared by beta radiation. The radioactive iodine that is not uptaken by the thyroid tissue is then rapidly eliminated via saliva, urine and sweat within a couple of days. The remainder is incorporated into thyroid hormone and is eliminated more slowly due to the hormone turnover and the radioisotope half-life (8 days for I131).

Following the administration of radioactive iodine, cats are housed in an isolation ward where they are housed until it is safe for them to return home. The isolation time varies according to the radioactive decay of I131 and the rate of uptake by the thyroid tissue but it is around 2 weeks long. All cats are checked for residual thyroid radioactivity to ensure that it is safe for them to be discharged.

During the isolation period, cats are monitored, fed and cleaned twice daily by qualified veterinary nurses and veterinary interns. Contact with the cats is avoided due to health and safety reasons and radiation exposure hazards.

Following discharge, cats will need to be housed strictly indoors and use a litter tray for approximately two weeks. During this time, contact with the owner is restricted to a maximum of 15 minutes lap-time once a day. Contact with children < 12 years of age, pregnant or breast-feeding women and women trying to conceive should be avoided.

No. At AMVS Feline Hyperthyroid Clinic all cases are individually assessed and doses are calculated based on a clinical scoring system that takes into consideration the severity of clinical signs and serum total T4 concentration. This system ensures that the lowest required dose of radioactive iodine is used avoiding unnecessary exposure risks for owners and staff members and reducing the risks of iatrogenic hypothyroidism.

All cases will be assessed and clinically scored prior to treatment by a team of RCVS, European and American recognised specialists in the Small Animal Internal medicine assisted by a team of capable veterinary specialists in training.

Around 95% of cases are cured after a single I131 dose. A small group of hyperthyroid cats, typically those with large goitres or very high thyroid hormone concentrations, will require a second treatment six months after the first. Finally, an even smaller percentage of cases will fail to respond to two treatments because they suffer from undiagnosed thyroid carcinoma that typically requires much higher doses of I131 (ten times higher than that required in adenomatous hyperplasia and adenoma cases).

If you wish to refer a case to our Feline Hyperthyroid Clinic please refer as normal through our online form or if you would like further advice please forward a brief covering letter together with the full written clinical history and copy of laboratory tests at the time of and after diagnosis to [email protected].

Our reception team will contact the clients to arrange a pre-treatment appointment for assessment of the cat’s suitability for radioactive iodine therapy. This is normally made within 2 weeks. This pre-treatment assessment will be carried out by one of our Internal Medicine Specialists who will review the case to ensure it is suitable for radioactive iodine treatment. If repeat bloodwork is necessary prior to treatment this can be carried out at the time of the assessment. 

This appointment will take place 2-6 weeks prior to radioactive iodine treatment and you will be contacted with an update and the treatment date.

Absolutely. Simply call our reception team on 01962 767920 and ask to speak to a member of the Internal Medicine Team or even better send an e-mail attaching the clinical history and bloodwork results to [email protected] and we will contact you as soon as possible to discuss. Furthermore, referral of all diagnosed hyperthyroid cats to our Internal Medicine Service for clinical staging should be considered to diagnose complications of hyperthyroidism (secondary cardiomyopathies, congestive heart failure) and co-morbidities (systemic hypertension, urinary tract infection, chronic kidney disease) that affect the case suitability for radioactive iodine treatment. Such clinical staging can be performed at your centre with our assistance as needed.

The following are instructions to be followed in all cases prior to referral to the feline hyperthyroid clinic in order for treatment to take place. Please read this information carefully to avoid unfortunate delays:

1. Cats need to be confirmed as hyperthyroid on the basis of an external serum total thyroxine (tT4) concentration above the laboratory reference range. Occasionally we will see and treat cats with high-normal tT4 measurements where hyperthyroidism has been confirmed by other methods (i.e. increased free T4), but these cases need to be discussed with one of our specialists prior to referral.

2. To minimise the risks of cats developing iatrogenic hypothyroidism and azotaemia after I131 treatment, we recommend that all cats are treated with oral anti-thyroid medication until 8 days before I131 treatment; patients receiving transdermal methimazole or an iodine-restricted diet (Hill’s prescription diet Y/D) should receive treatment until 14 days before treatment. Exceptions include cats with large goitres and severe disease that are at risk of thyroid storm after discontinuing treatment or cats that cannot tolerate medications (or cases where the owner is not able to pill the cat) or that will not eat the diet. Please discuss these exceptions with our internal medicine specialists.

3. Prior to referral, it is useful to assess the cats renal function based on  serum urea and creatinine concentrations (+/- SDMA), performed at times of euthyroidism (serum total T4 <35 nmol/L or in the low end of the laboratory reference interval) no later than 4-6 weeks prior to radioactive iodine treatment. Studies show that up to 50% of hyperthyroid cats will become azotaemic with treatment irrespective of which method (i.e. medical / surgical / radioiodine) is used. This occurs because the GFR is increased in the hyperthyroid state and the cat's true renal function can only be evaluated once GFR reduces with normalisation of thyroid hormone.
   • It is reassuring if the creatinine concentration remains in the reference range when the tT4 is reduced to <35 nmol/l and these cats are ideal candidates for radioactive iodine treatment.
   • If cats are minimally azotaemic but clinically improved on medical treatment, we will still consider treating them, but please call us to discuss the case before referral.
   • If the tT4 is still high, the dose may need to be increased and blood tests repeated - please contact us for further advice if this is happening.
   • If moderate to severe azotaemia is noted once the tT4 is in the normal range, these cats would not be considered ideal candidates for radioactive iodine treatment (which will irreversibly normalise GFR) due to risk of clinical deterioration. These cases may be better managed with anti-thyroid drugs or iodine restricted-diet. Please contact us to discuss.
4. The owners must be instructed to stop the anti-thyroid drugs or dietary therapy 8-14 days before their appointment at the feline hyperthyroid clinic.
5. Cats need to have been vaccinated for 'flu and enteritis' within the last three years.

However, our Internal Medicine team are happy to discuss cases where this has not been performed or if clients do not wish to wait for this process. Cats that are azotaemic whilst hyperthyroid (i.e., not on treatment) should ideally be treated with a non-definitive therapy to assess the renal function when euthyroid.

Yes. In order to confirm the success of treatment and identify the occurrence of iatrogenic hypothyroidism we recommend performing serum biochemistry, serum total T4, urinalysis and systolic blood pressure 30, 90 and 180 days after treatment and at least twice a year thereafter.

Most cats will be euthyroid or temporarily hypothyroid when they leave the Feline Hyperthyroid Clinic after treatment. In a few cases it may take up to 90 days for the thyroid hormones to decline within normal limits. Should the total T4 be elevated at day 180 post radioactive iodine treatment, the cat will be considered a non-responder and a second treatment (with or without prior scintigraphy scan) will be required.

It is rare but possible that the normal (spared) thyroid tissue will develop spontaneous hormone production after time (new hyperthyroidism). These tests can be done at AMVS or can be performed at the referring veterinary centre. Please contact us to discuss any results at [email protected].

Iatrogenic hypothyroidism is the most common complication of radioactive iodine treatment. 

It is possible for cats to be temporarily hypothyroid (based on low serum tT4 and high serum cTSH) up to 90 days post radioactive iodine treatment. In the absence of azotaemia or clinical signs (lethargy and severe weight gain) treatment is not necessary. However, if low serum tT4 and high cTSH are present 180 days post treatment, the cat has likely developed persistent hypothyroidism. Depending on the presence/absence of azotaemia and clinical signs associated with hypothyrodisim, supplemental therapy with levothyroxine may be required long-term.