The challenges of diagnosing feline hyperthyroidism

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You know the cat that has a total T4 that’s a little high but shows no clinical signs? Or maybe the total T4 is normal, but the clinical picture fits with feline hyperthyroidism, or comorbidities are complicating the diagnosis. Diagnosing feline hyperthyroidism can be straightforward, but often it’s not that cut and dry. At the Fetch DVM360® Conference in San Diego, held December. 2-4, 2022, Kelly A. St. Denis, MSc, DVM, DABVP, feline practice, discussed these difficulties and how additional testing can help diagnose this condition.¹

What is the thyroid’s role?

Thyroid glands sit on either side of the trachea. However, the cells can extend down into the thoracic inlet into the thorax. Through feedback loops through the hypothalamus and the pituitary gland, they regulate the body’s metabolism: respiratory rate, heart rate, nervous system activations, digestion rates, etc. Elevated levels lead to increased energy expenditure, fat breakdown, and protein turnover.

Feline hyperthyroidism

“Mature senior geriatric cats are at increased risk,” St. Denis said. “Probably as young as six years of age has been confirmed or reported in a published case report.” She said genetics such as mutations in the thyrotropin receptor (TSH receptor) may increase risk, but there’s not a large understanding of this notion, as may iodine and canned cat food cans. “Indoor cats, for some reason, seem to be at an increased risk, but again, this is an association [as opposed to a cause].”

“When we have passive hyperthyroidism, we know they'll lose body weight,” St. Denis said. Pet owners often attribute sudden weight loss to diet or more activity, but during your examination, an increased breathing rate or heart rate may be observed, or gallop rhythms changes in the heart, which could indicate the weight loss is not due to diet or activity.

However, “[cats] often maintain an ideal or overweight body-condition score,” she said. In fact, only one-third of cats are underweight as they are diagnosed with feline hyperthyroidism. This does depend on how advanced the disease is. Further, much of their weight loss will be associated with muscle condition.

Testing

Here is a list of tests, although not exhaustive, that St. Denis reviewed in her presentation, including preventative/routine testing as well as diagnostic testing:

Standard physical exam: check the thyroid for enlarged glands on the neck or the goiters; for senior care, the standard exam should include a full blood panel, total T4, urine analysis, etc.

Body condition scoring: Recommends WSAVA (World Small Animal Veterinary Association) because it is validated for body fat

Muscle condition scoring: Recommends WSAVA—it is only partially validated because it is subjective

Blood pressure: Hypertension and hyperthyroidism are intimately related. Also examine the retinas, as they are a hallmark of hypertension. Scan the back of the cat's eye for signs of hemorrhage. For example, cotton wool spots are the retina coming off the back of the eye. Examine the blood vessels in the back of the eyes, too.

IRIS staging of chronic kidney disease: Chronic kidney disease is highly interrelated to hyperthyroidism

Cardiovascular tests: Hyperthyroidism has a major impact on the cardiovascular system, so you will see changes in perhaps the heart rate (the heart might gallop) or hear new cardiac murmurs or changes in old ones, for example. It’s reported that 51% of cats with hyperthyroidism had cardiac abnormalities by echocardiogram, and cats with severe hyperthyroidism are more likely to experience thyroid toxic cardiac changes. Diagnostic testing may be needed.

Reference

St Denis KA. Cats don’t read textbooks: the conundrums of diagnosing feline hyperthyroidism. Presented at: Fetch dvm360® Conference; San Diego, California. December 2-4, 2022.