Hyperthyroidism is a very common disease in older cats and can produce a variety of symptoms. Although this disease has been reported in cats as young as six years of age, the majority of cases have been in cats over eight years of age. There is no apparent breed or sex tendency. The most common complaints in cases of feline hyperthyroidism are weight loss, polyphagia (increased appetite) and hyperactivity. Other significant signs that have been seen, although reported less frequently, include: polyuria (increased volume of urination), polydipsia (increased thirst), regurgitation, vomiting, diarrhea, increased fecal volume and increased respiratory rate or labored respiration.
Increased thyroid hormone levels cause increased energy metabolism and heat production in virtually all body tissues, resulting in increased appetite, weight loss, muscle wasting and increased body temperature.
Increased levels of thyroid hormones also interact with the nervous system, causing hyperexcitability, nervousness and muscle tremors.
Rapid overeating may cause regurgitation; this is generally seen in multiple-cat households. Vomiting may also result from a direct action of increased thyroid hormone levels on the chemoreceptor trigger zone, a center in the brain that causes vomiting.
Intestinal hypermotility associated with hyperthyroidism results in increased frequency of bowel movements and diarrhea.
The exact cause of polyuria and polydipsia in the hyperthyroid cat is unknown. Some patients may have concurrent chronic renal failure, which is very common in older cats. Physiologic changes in the kidney, due to increased renal blood flow, may result in kidney inability to concentrate urine normally, resulting in increased production of dilute urine and increased water consumption.
Increased levels of circulating thyroid hormones, over a long period, can result in heart changes known as thyrotoxic heart disease. Cats with this condition may exhibit signs of heart failure, with resultant rapid respirations or difficult breathing patterns.