New concepts in feline hyperthyroidism
Sarah M. A. Caney BVSc PhD DSAM(Feline) MRCVS, RCVS Specialist in Feline Medicine
Hyperthyroidism, the clinical syndrome resulting from excessive circulating levels of thyroid hormones - is believed to be the most common hormonal disease in cats. In the majority of cases, hyperthyroidism is caused by benign adenomatous hyperplasia of the thyroid tissue. The average age at diagnosis is 10-13 years. Common clinical signs include weight loss (usually in spite of a normal or increased appetite), polydipsia, irritability/hyperactivity, gastrointestinal signs (vomiting, diarrhoea) and coat changes. Abnormalities that may be detected on clinical examination include goitre, tachycardia, heart murmur and systemic hypertension. Treatment options include anti-thyroid medication, surgical thyroidectomy, radioiodine and now, a novel food: Hill’s Prescription diet y/d.
Advances in our ability to diagnose thyroid disease
Although the majority of hyperthyroid patients are easy to diagnose with a single total serum thyroxine (tT4) test, some cases can be difficult to diagnose:
1. Cats showing clinical signs compatible with hyperthyroidism but where tT4 results are within the reference range
Hyperthyroid cats may have tT4 results within the reference range for a couple of reasons. Firstly, levels of hormones fluctuate and in early disease this fluctuation may result in tT4 levels varying from normal to abnormal on occasions. Secondly, it is common for older cats to suffer from more than one illness. In hyperthyroid cats, presence of one or more concurrent illnesses (such as chronic kidney disease) can suppress tT4 levels into the reference range.
If the tT4 result is in the lower half of the reference range, hyperthyroidism is unlikely. However, if the tT4 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 tT4 measurement after a few weeks. Free T4 measured by equilibrium dialysis can be another useful diagnostic tool. An elevated free T4 (> 40 pmol/l) in addition to tT4 in the upper half of the reference range (> 30 nmol/l) is consistent with a diagnosis of hyperthyroidism (Wakeling et al 2008).
Use of the canine TSH (cTSH) assay can also be helpful. In patients with subclinical and occult hyperthyroidism, cTSH levels are low or undetectable. Therefore if tT4 levels are in the upper half of the reference range (> 30 nmol/l) and cTSH levels are low or undetectable (< 0.03 ng/ml), hyperthyroidism is diagnosed (Wakeling et al 2008). Other options for diagnosis of tricky cases include dynamic thyroid tests (T3 suppression, TSH/TRH stimulation test) and thyroid imaging (scintigraphy).
2. Diagnosis of subclinical hyperthyroidism
Subclinical hyperthyroidism is defined by having a tT4 within the reference range in combination with persistently low levels of thyroid stimulating hormone (cTSH < 0.03 ng/ml). These patients should be monitored more frequently for evidence of progression into overt hyperthyroidism which typically develops 1-3 years after subclinical hyperthyroidism is diagnosed.
3. Diagnosing hypothyroidism
Iatrogenic hypothyroidism is probably more common than currently recognised and is now known to be associated with a worse prognosis, especially regarding renal function and survival times (Williams et al 2010). Hypothyroidism can be diagnosed by measuring cTSH levels and/or by performing a TRH or TSH stimulation test. Cats with hypothyroidism have persistently low levels of tT4, elevated levels of cTSH and fail to respond to stimulation with exogenous TRH or TSH.
Medical treatment of hyperthyroidism should be titrated to maintain euthyroidism – the aim being to suppress tT4 levels to the lower half of the reference range in treated cats. In those cats where tT4 levels are persistently low following surgical treatment or radioiodine, cTSH measurement is indicated to confirm iatrogenic hypothyroidism. Thyroid hormone supplementation is recommended in confirmed cases of iatrogenic hypothyroidism.
Novel nutritional management options
Scientists from Hill’s Pet Nutrition have reported that exclusive feeding of an iodine restricted cat food is successful in managing hyperthyroidism (Melendez et al 2011a, Melendez et al 2011b, Yu et al 2011). The safety of this restricted level of Iodine was confirmed in another study where the Iodine requirements estimate determined for adult cats (0.46 mg I/kg) was higher than the current AAFCO recommendations (e.g. 0.35 mg I/kg), but lower than the 2006 NRC Iodine recommended allowance (Wedekind et al, 2011). Production of thyroid hormone requires iodine molecules therefore limiting the amount of iodine fed reduces the amount of thyroid hormone produced and released by the thyroid gland. Serum tT4 levels are reported to start decreasing within 3 weeks of the cat exclusively eating the food. No adverse effects have been reported although the same concerns over renal function would exist with this treatment as with all other treatment modalities for hyperthyroidism. Check-ups are recommended once the cat has been eating the food exclusively for 4, 8 and 12 weeks to assess the patient history, physical examination and laboratory findings (especially tT4 and renal function). More frequent check-ups may be indicated in cats where there are concerns of renal complications, as would be the case with alternative treatment modalities. Patient and owner compliance is essential to the success of this novel approach – even small deviations from the prescribed feeding can allow ‘escape’ of thyroid control. The food is only effective if used as the sole source of nutrition. Owners therefore need to be aware that their cat must not be allowed treats or other supplemental foods. Iodine levels in water may pose a problem to control in some areas; offering mineral water may be a safer option.
References available on request