Has Your Cat Become Irritable or Aggressive? Your Cat Could be Hyperthyroid!

Whenever an adult or aging cat shows a sudden change of attitude and becomes aggressive, hyperthyroidism must be high on the list of differential diagnoses. The classic symptoms associated with feline hyperthyroidism include weight loss despite an increase in appetite. Gastrointestinal upsets (such as vomiting or diarrhea) and increased thirst are also relatively common signs. Hyperthyroid cats commonly exhibit periodic restlessness, nervousness, hyperactivity, or hyperexcitably. Less commonly, they can become irritable and clearly aggressive.



Why do hyperthyroid cats develop these unpleasant behavioral signs?

In human patients, hyperthyroidism has long been known to produce a variety of psychiatric disturbances.  Human patients with hyperthyroidism commonly exhibit anxiety, nervousness, and hyperkinesia; they commonly develop an inability to sit still, continuously tapping their fingers, feet, or shifting their position frequently. They may experience insomnia, cognitive problems, phobias, panic attacks, and psychosis.  Irritability and emotional lability are common findings in these patients. They may lose their temper easily, burst into tears, and become hysterical without any apparent reason or justification.

Clearly, many of these psychiatric disturbances are difficult to recognize or characterize in cats.  As mentioned above, hyperthyroid cats are frequently restless and appear agitated. Pacing, circling or aimless wandering may occur; this may be similar to the confusion or anxiety that develops in human hyperthyroid patients.  The cats generally sleep less deeply and are more easily aroused than normal cats.  Other behavioral signs that can develop include increased vocalization, loss of litter box training (particularly defecating outside of the litter box), and diminished affection. 

Over two-thirds of hyperthyroid cats are reported to be nervous and hyperactive, and over half of the cats appear easily prone to “stress” or panic attacks. Up to 20% of hyperthyroid cats become extremely irritable, resist being held or touched, and can be outwardly aggressive.  Some cats, pumped up on thyroid hormone, develop a low tolerance for sudden touch or sound, spurring them to attach their owners or other pets.

Surprisingly, most owners do not bring their hyperthyroid cat to the veterinarian with the primary complaint of a behavioral problem, even when aggression is present.  Most of these cats remain basically very sweet when not under stress, so the development of severe aggression in the stress-free and safe environment of the cat’s house is extremely rare.

In general, severe aggressive behavior in hyperthyroid cats is most commonly precipitated by the stress of a visit to the veterinarian’s office. These cats may become aggressive and/or hysterical with handling and physical restraint required for the physical exam or blood collection. Therefore, the veterinarian is more likely to experience this aspect of the disease than the cats’ owners would at home. 

While behavioral concerns are not the usual presenting complaint, there are exceptions, and such was the case with Mattie, a 14-year old female domestic short-hair cat.   Most of the time Mattie behaved much as she always had, but every so often she would start to tear around the apartment, completely out of control.  At such times, she was most likely to attack both the other cats in the apartment as well her owners or human guests.  These attacks were typically quite vicious, and on several occasions she had drawn blood.  The only other abnormality the owner had noticed was that she had lost weight despite the fact that her appetite had increased. There was occasional vomiting, but no diarrhea, respiratory signs, or increased thirst.

On physical examination, Mattie’s body condition score was normal, weighing in at 9.5 pounds, but there was evidence of mild to moderate muscle wasting.  Initially, she tolerated physical examination fairly well with mild restraint.  Abnormalities included a rapid heart rate, mild heart murmur, and an enlarged thyroid tumor was palpable.  Near the end of the exam, however, Mattie showed us that the handing and restraint involved was more than she could tolerate; she became hysterical and attempted to bite and scratch the veterinary assistant. Fortunately, we were still able to collect a blood sample, and the subsequent finding of a high serum T4 concentration confirmed the diagnosis of severe hyperthyroidism.

Mattie was admitted to our Hypurrcat unit and treated with radioiodine.  After 2 weeks, Mattie's T4 had normalized and she was much calmer. Over the next month, the owners reported that Mattie’s hyperexcitability, agitated mental state, and aggressive behavior all resolved.

By far the most common medical cause of sudden aggression in cats is hyperthyroidism. So if you have an older cat that is showing behavioral issues of hyperactivity, irritability, or aggression, especially if your cat is losing weight despite a good or increased appetite, hyperthyroidism should be always ruled out. Treating hyperthyroidism with radioiodine, surgery, or antithyroid drugs resolves the behavioral problems within a few weeks in most cases. This allows the cat to return to their previous, calm, peaceful zen-like existence, sometimes napping for 16 or more hours a day!