Veterinary Information:
Hyperthyroidism in Cats

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Symptoms of Hyperthyroidism in Cats:
Symptoms can vary from mild to severe depending on the stage of this slowly progressive condition. The hyperactivity and increased appetite generally seen in these cats, often gives the owner the idea that their cat is doing very well for its age. This might prevent an early visit to the veterinarian until more serious symptoms develop. It is a multi-systemic disorder and not all symptoms might be apparent, depending on the stage of the disease. Other symptoms might be camouflaged or more difficult to identify due to the resemblance to other diseases.
-palpation of the thyroid gland in 80% of the cases (normally not).
-increased food intake / increased appetite (common)
-weight loss (common)
-behavioural changes: restlessness / hyperactivity / aggression (common)
-polyuria and polydipsia (pu/pd)**
-impaired capability to cope with stress (more aggressive)
-loose hairs / matting of hair / poor hair coat
-gastrointestinal signs: vomiting, diarrhoea, large volume feces*

More advanced stage:
Severe weight loss and generalised muscle wasting.
-signs of weakness / fatique / neck ventroflexion (possibly due to hypokalemia)
-breathlessness

Apathetic Hyperthyroidism (5% of the cases):
Most of these patients suffer from a concurrent disease.
-renal failure
-dyspnoea***
-cardiac disease****
-loss of appetite
-possible other neoplasias

*Gastrointestinal signs: Vomiting due to overeating is commonly seen shortly after feeding. Malabsorption, fatty feces and diarrhoea are due to intestinal hypermotility.
**Pu/Pd and Renal Disease: Thyroid hormones have a diuretic action. Renal blood flow and the GFR (glomerular filtration rate) are increased in hyperT4 cats. This results in an increased pu with a secondary pd.
Note: Cats suffering from both hyperT4 and chronic renal insufficiency might profit from a high GFR to prevent azotemia and uremia. On the other hand the increased GFR might contribute to the progression of renal disease. As chronic renal failure is common in older cats, serum creatinine or urea nitrogen should be checked before starting any treatment for the excessive amounts of thyroid hormones .
***Dyspnoea: dyspnoea, panting and hyperventilation can be due to generalised muscle weakness, stress and an increased production of CO2 due to a high metabolism in these cats. Sometimes cardiac failure can also play a role in developing dyspnoea.
****Cardiac disease: systolic murmers, tachycardia and gallop rhythms are common in hyperT4 cats. Thyrotoxic congestive heart failure is a less common sign. Cats can either develop a hypertrophic or dilative (rarer) cardiomyopathy as a result of volume overload.

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