Hyperthyroidism: Commonly asked questions. Part 1

Sarah M. A. Caney BVSc PhD DSAM(Feline) MRCVS

RCVS Specialist in Feline Medine
Chief Executive, Vet Professionals Ltd

Hyperthyroidism is a common condition, especially of older cats. Although many cases are straightforward to diagnose and treat, this is not always the case. What follows is the first part of an article that aims to answer some of the more commonly asked questions about the diagnosis and management of feline hyperthyroidism.


If the T4 is only slightly elevated, does the cat need treatment?

There are several situations in which this may occur:

  • False positive test result: Occasionally mild false positive elevations in total T4 can be encountered. Sometimes these are a laboratory error and may be more common with in-house T4 tests. In overweight cats there may be some thyroid resistance caused by the obesity and this has been hypothesised to potentially result in an increase in total T4.
  • Early hyperthyroidism: in early disease the T4 may be elevated before clinical signs have been noted by the cat’s owner.

If no goitre is present and/or there are no clinical signs of hyperthyroidism then a sensible plan would be to monitor the cat closely in the short-term, for example checking the patient history, bodyweight and physical examination once a month.

The owner should be educated regarding common clinical signs of hyperthyroidism and advised to return to you if concerned that any of these are developing. If clinical signs develop then repeating the total T4 test is indicated.

Diagnosis of hyperthyroidism is not an emergency since this is a disease which has an insidious onset and gradual progression. If in doubt over the validity of the T4 results received, consider sending serum to a reference laboratory for analysis.


What if I receive a normal total T4 in a cat which I suspect to have hyperthyroidism?


This usually occurs for one of three reasons:

  • In cats with mild or early disease, T4 levels may vary from within the reference range to just above this due to fluctuating levels of thyroid hormones.
  • Presence of concurrent illnesses (most commonly chronic kidney disease, CKD) can make diagnosis more difficult since total T4 levels can be suppressed into the reference range by the other illness – what is referred to as the ‘sick euthyroid’ syndrome.
  • Lastly, consider the possibility that your ‘clinical hunch’ is not correct and that hyperthyroidism is not responsible for your patient’s clinical signs. For example, differential diagnoses for weight loss in spite of a good or increased appetite also include inflammatory bowel disease, diffuse alimentary lymphoma, diabetes mellitus and exocrine pancreatic insufficiency.

If the total T4 result is in the lower half of the reference range, hyperthyroidism is unlikely. However, if the total T4 result is in the upper half of the reference range, hyperthyroidism remains a potential differential diagnosis. In these patients, a simple and often effective method of confirming the hyperthyroidism is to repeat the total T4 measurement after a few weeks. The author also encourages clinicians to use a reference laboratory for confirmation of hyperthyroidism when borderline or confusing results are obtained.

Free T4 measured by equilibrium dialysis can be another useful diagnostic tool. This test is highly sensitive in diagnosing hyperthyroidism although a small number of false positive results can occur, meaning that the free T4 test should not be used as a screening test for diagnosis of hyperthyroidism.

An elevated free T4 (> 40 pmol/l) in addition to total T4 in the upper half of the reference range (> 30 nmol/l) is consistent with a diagnosis of hyperthyroidism in cats showing clinical signs compatible with this condition.

If finances limit the extent of repeat blood tests then clinicians should consider monitoring the patient’s clinical status for example with monthly check-ups: weight loss and more noticeable clinical signs of hyperthyroidism are expected to develop in patients genuinely suffering from this condition. If noted, repeat total T4 measurement is recommended to confirm hyperthyroidism.


What is the best treatment for hyperthyroidism?


There are four broad treatment options for hyperthyroidism, two curative options and two reversible options:

  • Reversible options (lifelong treatment needed)
    – Antithyroid medication
    – Exclusive feeding of an iodine-restricted diet: Hill’s y/d
  • Potentially curative options
    – Surgical thyroidectomy
    – Radioiodine

None of these options are perfect – all have both advantages and disadvantages (see table below). The ideal treatment will therefore vary from patient to patient according to their individual circumstances. Where possible, curative treatment options should be pursued as these generally carry the best long-term prognosis.

Treatment considerations


Factor Medical management: thioureylenes (Methimazole, Carbimazole) Nutritional management: Hill’s Prescription dietTM y/dTM Surgical thyroidectomy Radioiodine
Is it possible to cure the condition with this treatment? No No Yes Yes
Other than the potential for reduced kidney function which can occur with all treatments for hyperthyroidism, are side-effects possible? Yes None reported Yes Yes
How common and how serious are the side effects? < 20% of cats have mild and transient side-effects

< 5% of cats have serious side effects

None reported Typically < 10% of cats suffer side-effects but these can be very serious < 5% of cats suffer side-effects and these are usually treatable
Is there a risk of permanent hypothyroidism? No – the dose of treatment can be reduced to resolve this No – if hypothyroidism is seen the food can be withdrawn Very rare (< 5%) Very rare (< 5%)
Is there a risk of recurrence? Yes Yes, if the cat eats other foods Yes Yes (much less common)
Will the cat need to stay in hospital? Not usually No Yes, usually < 3-5 days Yes, usually at least 1 week
Will the treatment be available in my location? Yes Available in most countries Yes Less common


When is best to collect blood samples for monitoring T4 levels?


The aim of medical management of hyperthyroidism is for total T4 levels to be reduced to the lower half of the reference range. For cats receiving once or twice a day treatment with antithyroid medication (orally or transdermally) the timing of blood collection for T4 measurement relative to medication administration is unimportant ie it does not matter when you collect your blood sample. There is some evidence that methimazole/thiamazole concentrates in the thyroid tissue and this may be one reason why timing of blood sampling is unimportant.

In some cases, optimal stabilisation of hyperthyroidism involves giving alternate day treatment (e.g. Vidalta – sustained release carbimazole – has a once a day or once every other day license). Alternatively, a different dose of methimazole/thiamazole may be needed on alternate days (e.g. 2.5 mg twice daily on one day, 2.5 mg once daily on the next, repeating).

Where possible, the author prefers to avoid alternate day protocols since it can be difficult and confusing for owners to remember what medication their cat needs.

There is no published data regarding optimal timing of blood sampling relative to alternate day antithyroid medication administration. The author prefers to collect samples on the day ‘off’ treatment (or lower dose of treatment). If the cat is doing well clinically and total T4 is in the lower half of the reference range on this sample then it indicates that the hyperthyroidism is well controlled.

The second part of this article on feline hyperthyroidism will be published next month. But if you need help with a tricky feline patient in the meantime please get in touch. We’d love to help!