Introduction
Hyperthyroidism is the condition of elevated and sustained metabolism caused by excessive amounts of circulating thyroid hormones. Affected cats normally exhibit a number of classic clinical signs, which usually are mild in the beginning and then increase in severity as the disease progresses.
Symptoms of Feline Hyperthyroidism
Cats with hyperthyroidism have abnormally high metabolism, which leads to a series of multisystemic clinical signs.
Increased Appetite with Weight Loss
One of the classic consequences of the increased metabolic rate caused by excessive circulating thyroid hormones is an increased appetite, which in turn causes increased consumption of food if it is available (called “polyphagia”). Oddly, despite this marked increase in appetite and presumably increased food intake, affected cats lose weight. Cat owners usually are baffled by their pet’s increased appetite, increased food intake and corresponding weigh loss. How can their cat eat so much but still lose weight? The answer is that hyperthyroidism dramatically increases the cat’s system-wide metabolism, which is what causes the high hunger with weight loss. The increased metabolic rate can also cause the cat to become much more active, or restless, especially in the early stages of the disease.
Increased Water Consumption
Cats with hyperthyroidism also become more thirst than normal and tend to drink an unusually large amount of water (called “polydipsia). As a result, they will urinate more frequently and in larger volume. Owners notice more frequent visits to the litter box, and the need to clean it more often than usual.
Gastrointestinal Signs
Affected cats frequently have gastrointestinal symptoms, including vomiting and diarrhea.
Respiratory Signs
Hyperthyroid cats often have difficulty breathing (dyspnea) and have an elevated respiratory rate (tachypnea).
Enlarged Thyroid Gland
The vast majority of cats with hyperthyroidism have palpably enlarged thyroid glands bilaterally (on both sides). Sometimes, owners can feel these enlarged areas each side of the trachea, under and behind the lower jaw along the upper throat. As these lobes of the thyroid become enlarged, and the cat loses weight, they become increasingly prominent.
Behavioral and Other Signs
Most affected animals become hyperactive, display excessive nervousness and/or become aggressive. Much less commonly, they may act weak and lethargic. Typically, they present in poor overall body condition, with an unkempt haircoat and sometimes thickened nails. They may become intolerant of heat and seek out cool places. On physical examination, the veterinarian often will find a racing heart rate (tachycardia), cardiac arrhythmias and/or murmurs, irregularities in the retina and dehydration.