Iodine Treatment Information
- Curative in 95% of cases
- Median survival times with I-131 significantly longer than with methimazole treatment alone.
- Minimal side effects
- Patients remain hospitalised until radiation levels have reduced to a safe level usually 7 - 9 days
- Extended stays can be accommodated
- Treatment is especially suited to cats who are:
- Under 15 years of age (to prevent neoplastic transformation over time)
- Patients intolerant to medical management or where compliance is an issue;
- Where surgery is not an option due to unacceptable anaesthetic risk or ectopic thyroid tissue.
The treatment is given by subcutaneous injection under mild sedation. Patients are hospitalised until radiation levels have reduced to a safe level (normally two weeks). We can accommodate extended stays if necessary (eg owner pregnancy). Most cases respond within two weeks of treatment, but sometimes the full response can be delayed for up to six months. Hypothyroidism can reoccur and treatment with thyroxine supplement is sometimes necessary.
We recognise that referrals can result in the loss of case continuity at the referring practice. Most of our patients have their pre-treatment investigations and post-treatment monitoring at their usual practice, with our support. This also benefits the client, as they only have to travel to us for the admission and discharge appointment. If you would like us to undertake the additional investigations (e.g. echocardiography by a cardiology advanced practitioner) this can be arranged.
Whilst radio-iodine treatment is suitable for most cases, it is not recommended in a small number of cases:
- Pre-existing IRIS stage 4 chronic kidney disease (IRIS stage 2 and 3 cases require discussion of risk:benefit)
- Patients with concurrent disease requiring medication that cannot be administered in food
- Thyroid carcinomas require significantly higher doses and can only be treated at 2 UK centres currently
Recommended pre-treatment protocol
The aim of the stabilisation process is to allow thorough assessment of renal function after any un-masking, together with true cardiac and hepatic assessment and to confirm no ongoing weight loss through other causes. Depending on initial findings prior stabilisation isn’t always necessary, especially if there is no evidence of azotaemia together with very mild increases in liver enzymes and no evidence of cardiac or other concurrent disease.
- Full clinical examination (including but not exclusively retinal exam, cardiovascular assessment, thorough abdominal palpation and thyroid palpation)
- Haematology, biochemistry and total T4 (external laboratory) and ideally urine specific gravity
- Medicate with carbimazole/methimazole/thiamazole/yd diet
- Once time elapsed to stabilise, repeat haematology, biochemistry and total T4 (external lab) together with urine specific gravity and ideally blood pressure
- Repeat full clinical examination when tT4 is in the lower half of the reference range.
- Additional tests may be advised depending on findings
- Cardiac investigation if significant murmur, tachycardia or arrhythmia persists
- Imaging if concern re concurrent neoplasia etc
- Fasting bile acid, or bile acid stimulation test if no improvement in liver enzymes
- Further urinalysis including culture
- FeLV/FIV testing if high risk patient
Cats should be up to date with their vaccinations prior to treatment. Once the treatment date is confirmed, medication should be stopped usually 10 days (y/d diet 14 days) before. Routine worming and flea medications should be administered 1 week before treatment. A repeat tT4 (external lab) will be required 5 days before treatment- this is used to aid dose calculation.