Plus-Hex CLINICAL
Introduction
Feline hyperthyroidism ( FHT ) was virtually unheard of in the 1980s [ 1 ] , but its incidence has increased in recent decades . It is currently the most common feline endocrine disorder , affecting approximately 10 % of cats aged 10 + years [ 1 – 3 ] .
Of these cases , 95 % are due to benign adenoma growth or adenomatous hyperplasia of the thyroid gland , and the remaining cases are caused by malignant carcinoma [ 2 ] .
The exact cause of FHT is unclear , but it is likely to be multifactorial . Genetics , environmental factors , husbandry , lifestyle and exposure to goitrogens ( compounds that interfere with the production of thyroid hormones ) are all linked to the disorder [ 1 , 2 ] , although these links remain non-causal in the absence of definitive evidence [ 1 ] .
Treatment options for FHT
There are currently four treatment options , all with advantages and disadvantages associated with their use :
• Management with oral / transdermal medications ( carbimazole or methimazole / thiamazole )
• Unilateral / bilateral surgical thyroidectomy
• Dietary management ( Hill ' s Prescription Diet y / d )
• Radioactive iodine therapy ( RAIT ) with iodine-131 ( I-131 ).
This article will focus on RAIT .
Advantages of RAIT [ 1 , 2 , 4 ]
• It is curative , with a success rate of ̴95 % with a single treatment
• It has few adverse effects
• There is no need for daily medication or general anaesthesia
• It is the treatment of choice for thyroid carcinoma
• It addresses all affected tissue ( e . g . ectopic adenomatous tissue in the neck or thorax )
• It targets abnormal tissue while sparing normal thyroid tissue
• Minimal testing / monitoring is required once stable euthyroidism is achieved ( but , due to the advanced age of most affected cats , routine monitoring for other age-related conditions is still advised ).
Disadvantages of RAIT
• A lump sum is payable for the therapy , as opposed to paying for medications and monitoring over time
• Hospitalisation / isolation is required during treatment , followed by a period of restricted handling at home
• Its availability is limited by the number and location of clinics licensed to provide RAIT
• A small proportion of cats remain hyperthyroid after a single dose ( ̴4 %) or become hypothyroid ( ̴2 %)
• It might not be suitable for cats with significant and / or unstable comorbidities
• It should be undertaken with caution , with additional consideration given to isolation and handling , particularly by children and pregnant women , in the immediate post-discharge period .
I-131 properties and mode of action
I-131 is a by-product of nuclear fission and a radioisotope – a type of atom that is unstable due to excess nuclear energy . It has an atomic mass of 130.9 daltons and a physical half-life ( t-1 / 2 ; the length of time taken for its radioactivity to decay to half its original value ) of 8.02 days [ 5 , 6 ] . Its biological half-life ( in blood plasma ) is 1.5 – 4 days [ 1 ] . Owing to the decay process , the radioactivity of a radioisotope after 10 half-lives is approximately 1 / 1,000 of its original radioactivity [ 7 ] .
The half-life of a radioisotope is ‘ the time interval for a particular radioactive material to reduce ( decay ) its radioactivity by half ’ [ 7 ] .
After being administered orally or by subcutaneous injection , I-131 is absorbed into the body via the gastrointestinal tract and bloodstream , and distributes in the extracellular fluid [ 8 ] . It is then almost exclusively taken up and concentrated by abnormal thyroid tissue while leaving normal thyroid cells unaffected .
Around 90 % of I-131 decays into a more stable form ( xenon-131 ), via the emission of beta ( β ) particles , which penetrate 1 – 2 mm into surrounding tissues and are the means by which overactive thyroid tissue is destroyed [ 3 ] . The remaining 10 % of decay occurs via gamma ( γ ) ray emission , which can penetrate further due to its short wavelength . These gamma rays are a safety hazard for people who come into close contact with treated cats .
Volume 38 ( 5 ) • October 2023
37