What to Look For

Paying close attention to symptoms of hyperthyroidism can be the difference between life and death.

Top 10 Hyperthyroidism Symptoms

Starting with the Most Common

Weight Loss

This is the most common symptom of hyperthyroidism, affecting roughly 90% of cats. This disease causes an increase in your cat’s metabolic rate. As soon as those food calories come in, they get burned, even when the cat is at rest. No matter how much your cat eats, weight loss is still imminent. The deterioration of muscle can lead to death from starvation, if not monitored and treated.

Lumps on the Neck

Your cat’s thyroid glands can become enlarged, or you may notice some bumps around the gland in neck area. While this is not always indicative of hyperthyroidism, it should always be examined.

Increased Appetite

About 50% of cats with hyperthyroidism show an increase in food consumption. You may find your cat is constantly craving another meal, and can eat up to twice its regular portion. This is due to an increase in their metabolic rate, which has them yearning for more calories to fill the void. Unfortunately, they are never able to offset the calories burned and continue to lose weight.


Vomiting is prevalent in almost half of cats with hyperthyroidism. This is likely a result of their increasingly overactive eating habits, and vomiting is primarily observed shortly after meals. Basically, your cat eats so quickly and ferociously, that their stomach becomes jam-packed, causing everything to be released.

Increased Thirst and Urination

Increased thirst is generally a result of some form of kidney disease that is caused by excessive production of thyroid hormones. Over a third of cats with the disease are seen at the water bowl with increasing frequency. The constant urination is a direct result of the excessive drinking. Simply monitoring your cat’s dish, and noticing that it is becoming empty with more regularity, is the easiest way to spot this symptom.


A third of cats will exhibit increased bouts of anxiousness and irritability. They become more demonstrative and aggressive in their day to day actions. Some moments they seem like they were shot out of a cannon, as increased levels of hormones make it difficult for them to unwind and calm down. This interferes with normal sleep cycles, and makes your cat seem incessantly “out of it.”

Disheveled Hair Coat and Nails

Cats with hyperthyroidism neglect grooming and no longer care for their coat. This is evident in a third of cats with the disease. You will notice a grungy and matted look to your cat’s hair coat. Their nails also grow thicker and more quickly. Eventually they become significantly more frail, and begin to crack or chip frequently.

In some cases, cats go in a completely different direction, notably overdoing their grooming. Constant licking and scratching leads to bald spots or rashes (known as miliary dermatitis).


20% of cats with hyperthyroidism experience diarrhea. This is caused by the speed in which food is passed through their digestive tract. As a result, there are more recurring bowl movements. The stools become quite foul smelling, and are in bigger chunks than they were previously.

Occasional Breathing Difficulties/Panting

Some cats develop a rapid respiratory rate and other respiratory irregularities. They become increasingly intolerant of heat, and prefer to find shade or remain indoors. In extreme cases, it intermittently becomes difficult for them to breath, especially in warmer climates. Owners observe episodes of panting caused by shortness of breath.

These issues can lead to heart problems, and should not be taken lightly. Since the symptoms tend to occur sporadically, many owners are likely to ignore them. Hyperthyroidism increases the cat’s overall body temperature, and all these signs should alert you that there is a serious problem.


In rare and primarily advanced cases, the hyperactivity turns 180 degrees. This is seen in about 10% of cases. Your cat shows signs of listlessness and fragility. Due to the higher metabolic rate caused by increased thyroid hormone production, cats with hyperthyroidism have nutritional deficiencies. They are less likely to run around or find joy or happiness in little things as they used to. In this stage, highly dangerous other symptoms are accompanied, such as loss of appetite. When combined with weight loss, this can be fatal.

Percentage of Cats Showing Symptoms