Information for Veterinary Surgeons
The Feline Hyperthyroid Clinic is a dedicated service providing radioiodine (I-131) therapy for cats with hyperthyroidism. I-131 is the treatment of choice for most cats with hyperthyroidism.
Advantages of Radioiodine Therapy
- More than 95% of cats are cured of hyperthyroidism following a single treatment.
- Life expectancy is doubled with radioiodine treatment compared to treatment with methimazole.
- Thyroid neoplasia is progressive – the prevalence of thyroid carcinoma rises from under 2% at initial diagnosis to around 20% with long-term methimazole treatment.
- Owners no longer have to administer daily medications to their cats.
- Costs between long term medication and radioiodine treatments are comparable when monitoring and medication checks are taken into account.
- Cats do not have to be on methimazole or any other medication prior to radioiodine treatment – radioiodine can be given as soon as the disease is diagnosed.
Overview of Radioiodine Therapy
Radioiodine (iodine-131) therapy is administered as a single subcutaneous injection, with the dose calculated on an individual basis. The iodine is selectively taken up by the thyroid tumour, where it emits beta radiation that destroys abnormal cells while sparing surrounding structures. Any iodine not taken up by the thyroid gland is eliminated quickly from the body – primarily through urine, saliva, and sweat over the first couple of days. The remaining radioactivity leaves the body more gradually, through two processes: the natural turnover of thyroid hormone (which incorporates the I¹³¹ and is then excreted in the urine), and the physical decay of the isotope itself, which has a half-life of eight days.
Patient Selection and Contraindications
Cats referred for treatment should have a confirmed diagnosis of hyperthyroidism, typically based on an elevated total thyroxine (T4) concentration alongside compatible clinical signs. In cases where the diagnosis is less clear, for example, borderline or high-normal T4 concentrations, discussion prior to referral is encouraged.
It is usually not necessary to medically stabilise otherwise healthy cats with anti-thyroid medication prior to radioiodine treatment, if they are referred quickly. There are some exceptions to this:
- Cats with moderate to marked azotaemia (late IRIS stage 2+). In the hyperthyroid state, glomerular filtration rate (GFR) is artificially elevated, masking true underlying renal function. It is only once thyroid hormone levels normalise and GFR falls that a reliable picture of renal health emerges. Administering anti-thyroid medication to azotaemic cats will give an indication of how the cat will be after radioiodine treatment.
- If the clinical signs are not classic for hyperthyroidism, anti-thyroid medication should be trialled to assess if the clinical signs are caused by the hyperthyroidism.
- Most cats will benefit from having their hyperthyroidism cured.
Referrals can be made by submitting the cat’s clinical history, recent laboratory results (to include haematology and biochemistry, urinalysis and blood pressure), details of current medications, and client contact information via our website or email. Once received, the case will be reviewed and the owner contacted directly to arrange an initial assessment and provisionally schedule treatment. As demand for radioactive iodine therapy is consistently high, early referral is advisable where possible.
Most cats are suitable candidates for radioiodine therapy; however, those that are very aggressive or have significant or poorly controlled co-morbidities requiring regular daily medication may not be appropriate for treatment. Radiation safety legislation prevents close contact with these patients, so treatment options are limited once they have received radioiodine. Please contact us for individual case advice. Cats should also be up to date with vaccinations prior to admission.
Prior to the appointment
Anti-thyroid medications and low iodine diets need to be stopped two weeks before the appointment. Specific instructions are provided directly to the owner to ensure appropriate preparation. Gabapentin can be a useful anxiolytic in fractious cats given 60-90 minutes prior to their appointment.
Duration of Hospitalisation and Isolation
The duration of hospitalisation depends on the administered dose of radioiodine and the rate at which radioactivity declines. This varies, but most cats remain hospitalised for approximately three to ten days.
Post-discharge requirements
After discharge, owners are given clear guidance regarding temporary precautions to limit radiation exposure to others. These include keeping the cat indoors for a minimum of 1 week after discharge and restricting close contact to people, particularly with children and pregnant individuals. Used cat litter must either be flushed down the toilet or stored in an outside bin for a period of 6 weeks prior to disposal in the council bin. If the owners cannot meet these requirements, then the cat must stay for longer in the practice.
Post-Treatment Monitoring and Follow-Up
Follow-up care is an essential component of treatment. We recommend that cats are reassessed at the referring practice approximately four weeks after discharge. This visit should include a clinical examination, measurement of total T4, assessment of renal parameters, urinalysis and blood pressure measurement. Further monitoring at six months is advised. These assessments allow confirmation of treatment success, identification of transient or persistent hypothyroidism, and early detection of any emerging renal disease.
We are always happy to discuss individual cases or provide guidance at any stage, whether prior to referral or during follow-up.
Additional Resources
2016 AAFP Guidelines for the Management of Feline Hyperthyroidism