Hyperthyroidism: radio-iodine treatment in Cats (Felis) | Vetlexicon
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Hyperthyroidism: radio-iodine treatment

ISSN 2398-2950


  • Best method of treating human and feline hyperthyroidism Hyperthyroidism.
  • Fewest complications and highest success rate (94-98%).
  • Uses a radioactive isotope of iodine - iodine-131 (I-131).
  • Suitable for cats that have had previous therapy (thyroidectomy Thyroidectomy, methimazole Methimazole).
  • High-dose radio-iodine treatment has been used successfully to treat cats with thyroid carcinoma Thyroid gland: neoplasia.
  • Main contra-indication is the presence of intercurrent disease that requires daily monitoring/treatment.
  • Licensed facilities also offering radio-iodine treatment currently available in the UK are the Animal Health Trust, the University Veterinary Schools at Bristol, Edinburgh, Glasgow, London and Royal Veterinary College, Barton Veterinary Hospital, Bishopton Veterinary Group, Pride Veterinary Centre, Rowe Veterinary Referrals, and The Hyperthyroid Cat Centre. Please contact these centers for relevant details.
  • For centers that offer radiotherapy treatment for feline hyperthyroidism in the USA, please visit:  http://www.veterinarypartner.com and search for radiotherapy facilities.

Cats with pre-existing renal disease may deteriorate after treatment for hyperthyroidism by any method. Radio-iodine offers no significant advantage over medical or surgical therapy in this respect.Print off the owner factsheet Cancer treatment - radio-iodine treatment for thyroid cancer  Cancer treatment - radio-iodine treatment for thyroid cancer to give to your client.


  • Normal thyroid follicles are suppressed in the hyperthyroid cat by the overproduction of thyroid hormones by the hyperplastic, adenomatous or, more rarely, adenocarcinomatous tissues.
  • Thyroid cells (normal, hyperplastic, adenomatous and adenocarcinomatous) will take up radio-iodine as well as stable iodine but the suppressed normal cells do not take up iodine in the hyperthyroid cat.
  • Over 50% of injected iodine will be concentrated within the thyroid gland.
  • Beta particles produce local (2 mm range) irradiation which leads to destruction of tissue.
  • Following destruction of the hyperfunctional thyroid tissue, normal tissue will gradually recover its function.
  • I-131 produces both beta and gamma radiation.
  • Beta radiation penetrates tissue to a depth of no more than 2 mm (average depth is 0.4 mm) and causes the most damage.
  • Gamma radiation causes less local damage as it passes through tissues almost unaltered.
  • Cost of treatment should be weighed up against long term medical management or surgical treatment.

Radiation safety

  • Iodine solutions vaporize readily and up to 10% of radioactivity in a vial can be in the form of iodine vapor. Such solutions must therefore be handled in a dedicated laminar flow safety cabinet. Conventional fume hoods may not provide sufficient operator protection.

Beta radiation

  • Small external hazard as does not penetrate tissue but if radio-iodine is accidentally inhaled or ingested then beta radiation poses an internal hazard.

Gamma radiation

  • External hazard to workers in proximity of cats.
  • Significant proportion of radiation penetrates lead up to 25 mm thickness.
  • Effective shielding is therefore impossible when injecting cats.
  • The only practical methods of reducing exposure are to use the lowest effective dose (requires a method of dose calculation) and to keep the time spent performing injection to an absolute minimum.
  • Major hazards are inhalation of aerosols and accidental self injection.
  • Contamination can be detected using standard monitoring equipment.

Indications / contraindications


  • Radioactive iodine is gold standard treatment for feline hyperthyroidism. All treatments are associated with potential complications. Medical therapy may not be tolerated by some cats due to difficulties in administering tablets on a daily basis and adverse effects of the drugs Methimazole (transient vomiting, anorexia, and diarrhea along with more serious complications such as blood dyscrasias and hepatopathies).
  • Used to treat cats with hyperthyroidism including:
    • Unilateral or bilateral disease (whether cervical or intrathoracic).
    • Cats in which previous thyroid surgery has, or may have, failed to preserve parathyroid glands.
    • High dose treatment in cats with thyroid carcinoma Thyroid gland: neoplasia
    • Cats with intra-thoracic nodules or metastatic disease in which disease is therefore no amenable to surgery.
  • If you are considering referring a cat for this treatment read the information supplied by the referral institution. 


  • Should not be used in:
    • Cats with intercurrent diseases, eg epilepsy, diabetes mellitus, that require daily therapy.
    • Cats with intercurrent diseases that significantly reduce life expectancy, eg neoplasia, FeLV.
    • Cats with significant renal dysfunction (azotemia and urine specific gravity of <1.025).
    • Cats with infectious diseases which might spread within isolation unit.
    • Cats in which anesthesia is not possible.
      Cats with hyperthyroidism are generally geriatric and therefore incidence of concurrent disease is high. It is important full investigation is performed before treatment as they cannot be removed from the treatment facility once treatment has started.Cats must remain in facility for 3 weeks and restrictions continue once they have been discharged. This can be stressful for cats and owners.


Cats must be assessed before treatment to ensure suitable for long term.Anti-thyroid medication is withdrawn about 1 week before admission.

  • Calculate dose of I-131 required by adding points from a scoring system.
    For confirmed adenocarcinomas the dose should be increased and may require repeated administration.

Thyroid hormone concentrations

  • Award points from 1-5:
    • Total T4 60-80 nmol/l = 1 point.
    • Total T4 80-100 nmol/l = 2 points.
    • Total T4 100-150 nmol/l = 3 points.
    • Total T4 150-400 nmol/l = 4 points.
    • Total T4 400+ nmol/l = 5 points.

Clinical signs

  • Award points from 1-5:
    • No significant signs = 1 point.
    • Severe weight loss, GI signs or behavior changes = 5 points.

Size of goitre

  • Graded from 1-5:
    • Small (<0.5 mm) = 1 point.
    • Very large (or not detectable) = 5 points.
      If not detectable then assume is intrathoracic and large - unlikely to harm cat by giving more I-131.
  • Add three scores together and the total number of points are then used to calculate the dose:
    • 3-9 points give 80-120 MBq.
    • 9-12 points give 120-150 MBq.
    • 12-15 points give 150-200 MBq.
      NB: some authorities give a fixed dose, others calculate doses based on scintigraphy using Tc 99m.
  • I-131 should be sent from dispensary, eg local hospital, by licensed transport in a lead lined casket labeled as containing radioactive material and once dose has been drawn up the syringe must be kept in lead lined syringe carrier.
  • Protective clothing (surgical gown or white coat with a wrap-over collar, several pairs of non-porous surgical gloves and a face mask) is required.
  • Cat may be sedated for subcutaneous or IV injection. Ketamine Ketamine and midazolam Midazolam are a safe combination but other agents Sedation or sedative protocol may also be satisfactory.
  • Cat is kept in isolation until no longer radioactive.
    During isolation contact with the cat should be minimized.
  • All waste (including used vials, needles, syringes, gloves bodily excretions and unused food) should be collected in thick plastic bags and placed in radioactive waste bins.
  • Radioactive waste bins and empty caskets should be disposed by a licensed contractor.
  • Tablets, eg atenolol Atenolol, may be placed in animal's food but not in animal's mouth.
  • In the rare event of a cat dying in isolation unit the body must be retained until it is no longer radioactive.
  • Side effects of I131 are rarely reported.
  • Monitor renal function and total T4 concentration at 1,3, 6 and 12 months. Some cats biochemically hypothyroid at time of discharge but treatment is rarely required.
  • Most cats return to euthyroid state within 4-6 weeks of treatment (but may take up to 3-6 months).
  • Unmasking of chronic renal failure may occur post-treatment for hyperthyroidism.


  • 95-99% cure rates from first injection although full response may take up to 3 months.
  • Most of the remainder are cured by second injection.
  • Recurrence can occur but is rare. When recurrence does occur can be 3 months to 3 years.
  • Complication rate <1%.
  • Clinical signs usually improving within 2 weeks of injection.

Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Harvey A M, Hibbert A, Barrett E L et al (2009) Scintigraphic findings in 120 hyperthyroid cats. J Feline Med Surg 11 (2), 96-106 PubMed.
  • Hibbert A, Gruffydd-Jones T, Barrett E L et al (2009) Feline thyroid carcinoma: diagnosis and response to high-dose radioactive iodine treatment. J Feline Med Surg 11 (2), 116-124 PubMed.
  • Nykamp S G, Dykes N L, Zarfoss M K et al (2005) Association of the risk of development of hypothyroidism after iodine 131 treatment with the pretreatment pattern of sodium pertechnetate Tc 99m uptake in the thyroid gland in cats with hyperthyroidism: 165 cases (1990-2002). JAVMA 226 (10), 1671-1675 PubMed.
  • Chun R, Garrett L D, Sargeant J et al (2002) Predictors of response to radioiodine therapy in hyperthyroid cats. Vet Radiol Ultrasound 43 (6), 587-591 PubMed.
  • Peterson M E, Becker D V (1995) Radioiodine treatment of 524 cats with hyperthyroidism. JAVMA 207 (11), 1422-1428 PubMed.
  • Slater M R, Komkor A, Robinson L E et al (1994) Long-term follow-up of hyperthyroid cats treated with iodine-131. J Vet Radiol Ultrasound 35 (3), 204-209 VetMedResource.

Other sources of information

Further information

The Webinar Vet - Common challenges and practical solutions in feline hyperthyroidism: Treatment