The Listless Pet

The Thyroid and Anxiety, Aggression, and Mood

The cure of the part should not be attempted without the cure of the whole.

—PLATO

A few years back, I listened to a woman speak on NPR about some issues she was having with her memory and mood. She kept losing her car keys, she said, which was not normal for her, and she was often tired. In the most general terms, she simply did not feel right. She knew something was wrong and, for some reason, suspected her thyroid was out of whack. Her doctor’s test for thyroid dysfunction was negative, and he reported that all values were “within normal limits.”

The woman did not quit in her quest to find the source of her difficulties. She asked to be referred to an endocrinologist and her doctor obliged. On rerunning the tests, the endocrinologist diagnosed subclinical hypothyroidism, a borderline state of thyroid dysfunction in which thyroid hormones are within the normal range but suboptimal for normal functioning.

The specialist treated the woman with thyroid hormone replacement therapy, prescribing levothyroxine. Her condition immediately improved. No more funky mood swings, no more memory loss, no more tiredness and irritability. The transformation was profound.

With impressive communication skills and determination, this woman managed to help the medical profession help her. Unlike a doctor interviewing such a patient, veterinarians cannot question animals about what ails them. “Do you remember where you buried your bone?” isn’t a viable clinical approach to check for memory loss in dogs, nor is “Are you worried all the time?” when seeking signs of anxiety in cats.

Instead, we veterinarians must figure out our patients’ health from observing them. And we can miss the nuances in the presentations of our patients. A condition such as borderline hypothyroidism, for example, is difficult to nail down in people, much less pets. People with depression, anxiety, and other psychiatric problems often have abnormal or low blood levels of thyroid hormones. And treating the thyroid problem can lead to improvements in mood, memory, and cognition.

Hormones secreted by the thyroid gland act as the central nervous system’s traffic cops, functioning as neuromodulators and neuroregulators. Thyroid hormones also influence other neurotransmitters like serotonin, norepinephrine, and dopamine, helping to control mood and behavior. Low levels of thyroid hormones lead to decreased serotonin, decreased norepinephrine, and reduced dopamine levels in the brain.

None of these effects is good. Low levels of serotonin are associated with depression, sleeplessness, and anxiety. Low levels of the catecholamines, norepinephrine, and dopamine, both “go” neurochemicals, set a person up for tiredness and depression. Dopamine is involved in the brain’s natural reward system. With too little of it around, the impetus to engage in pleasurable activities is muted.

Most medical professionals quizzed about the mental status of someone with hypothyroidism would invoke words like “depressed, lacking energy, lethargic.” That is true of full-blown hypothyroidism, often referred to as “frank” hypothyroidism. Signs of the borderline condition, on the other hand, are more subtle. They include memory impairment, anxiety, as well as behaviors that derive from these impairments. Correction of borderline or low thyroid status often completely reverses this literally depressing situation.

In people and pets, borderline hypothyroidism can also lead to paradoxical effects of agitation, paranoia, and aggressiveness, which can be confusing for physicians and vets, who can easily miss the right diagnosis. Is Fido just getting old, or is there something physically wrong that is causing her listlessness? Making the leap from behavioral condition to a medical issue underlying it can be tricky, but there is often a direct link. Other common medical problems in pets, such as partial seizures and Alzheimer’s disease, have behavioral consequences, and animals and people who are ill may retreat into themselves, act depressed, or become extra needy. The psychosomatic effects of excessive anxiety in pets can cause very real physical problems, as in the case of irritable bowel syndrome and acral lick dermatitis.

Veterinary students have an acronym to help them to remember the various medical conditions they may encounter. It is referred to as DAMN-IT:

• D = Degenerative or Developmental

• A = Anomalous or Autoimmune

• M = Metabolic, Mechanical, or Mental

• N = Nutritional or Neoplastic

• I = Inflammatory, Infectious, Ischemic, Immune-mediated, Inherited, Iatrogenic, or Idiopathic

• T = Traumatic or Toxic

Any of these conditions may affect behavior. A prime example is a metabolic one, hypothyroidism, the most common genetic disease of purebred dogs. It is the number one health concern in six of the seven AKC-recognized breed groups. Since hypothyroidism is so prevalent, then veterinarians and behaviorists should reasonably expect to encounter not only frank hypothyroidism on a regular basis, which we do, but also borderline hypothyroidism.

The first time I encountered hypothyroidism contributing to a psychological condition in dogs was many years ago. A family of Afghan hounds who were regularly shown by their owners as fine examples of the breed had anxiety problems. From a young age, these dogs went through mock show exercises to prepare them for real events, but at some point between nine and eighteen months of age, some of the Afghans began showing extreme anxiety when approached or examined by the show judge. Before a certain age the dogs were perfectly fine in the mock show situation. Then, as they crossed an age threshold, it was as though a light switch had been thrown. They became anxious.

Blood tests showed that the Afghans had low-to-borderline thyroid hormone levels. When these dogs were treated with a supplementary thyroid hormone, levothyroxine, their anxiety dissipated. All was well on the exhibition circuit.

Hypothyroidism gradually became recognized by other experts as causing anxiety and other anxiety-related behaviors. It’s a great feeling, when you can sense the scientific community closing in on a problem. Soon word on the street among dog trainers and veterinarians was that borderline-to-low thyroid hormone levels were associated with anxiety and aggression.

With all this in mind, I started to look more closely into behavior cases that were presented to our clinic at Tufts. Dogs who were anxious or aggressive, especially when those behaviors were associated with subtle physical signs of hypothyroidism, now prompted me to perform a thorough thyroid hormone analysis.

Clinical signs that tipped me off to a possible diagnosis of borderline hypothyroidism included excessive shedding, premature graying of the muzzle, a sad facial expression, drooping lower eyelids, and slow hair regrowth of previously shaved areas. Weight gain was another indicator. Dogs with borderline thyroid felt cold all the time and sought out sunlight, heaters, or other warm areas. They exhibited low body temperature, slow resting heart rate, and skin thickening, especially in areas of skin-to-skin contact such as armpits. Skin thickening also causes drooping in the facial region, contributing to a “sad” expression.

For patients with these symptoms, I ordered a blood test and, if results indicated that thyroid levels were borderline low range, I treated the dogs with hormone replacement. Any ranking that was in the bottom tenth percentile of the normal range was cause for concern. Many dogs improved dramatically in as little as five days. Other times, improvements took as long as six weeks.

One case that I clearly remember involved a German shepherd who displayed aggressive behavior to puppies, other adult dogs, and to strangers, but showed absolutely no physical signs of hypothyroidism. The owner had been to vets and trainers everywhere, done everything she knew she should, and spent all kinds of money in vain attempts to resolve her dog’s behavior problems.

I decided to check the dog’s hormone blood levels in case the previous examinations had missed something. Out of the 177 AKC-recognized breeds, German shepherds rank number seven on the hit parade for hypothyroidism. I did not want to leave any stone unturned with this dog. The owner was desperate.

The dog’s thyroid levels came back in somewhat low, but still in what formerly was deemed an acceptable range. I was in the same position as the woman I had heard about on NPR, whose thyroid hormone levels were officially medically normal. I initiated treatment with thyroid hormone replacement anyway.

The dog responded brilliantly and became a changed animal. Five days after his thyroid hormone treatment began, the owner took the dog on a walk. The shepherd played with a puppy, engaged in friendly tug-of-war with another dog, and allowed himself to be approached by strangers. The owner emailed to tell me that she cried tears of joy after that walk. After two years of input from multiple trainers and veterinarians, this was the first concrete progress she had seen. And the dog’s improvement lasted.

Another dog I treated was violently aggressive to all other dogs. As with the German shepherd, I checked his thyroid levels, found them to be lowish and treated him with thyroid replacement therapy. That was all this dog needed. His aggression toward other dogs completely disappeared, simply by correcting his thyroid status.

I wrote up a case report, documenting three aggressive dogs responding well to thyroid hormone replacement, which was featured as the Behavior Case of the Month in the Journal of the American Veterinary Medical Association. I hoped this might lead some of my colleagues to pursue a similar diagnosis with their behavior cases, but there were skeptics. Some veterinarians simply doubted the existence of borderline hypothyroidism as a cause of canine behavioral problems and aggression. Others saw the issue in binary terms, either/or, black or white—no gray scale allowed, if you please. Such binomial thinkers recognized only two possible findings, normal and low thyroid status. There would be no borderline interpretations for them.

It is easy to recognize the robustly normal thyroid state in a dog with no physical problems, no behavioral problems, and thyroid hormone levels in the mid- to upper end of the normal range. Also obvious is frank hypothyroidism, since blood work will show hormone levels to be well below the lower limit of normal. These dogs often display other signs, such as being overweight, lethargic, apathetic, or having bilaterally symmetrical hair loss. Their condition can be diagnosed from the top of a double-decker bus with a telescope turned the wrong way around.

I don’t believe that there is only door number one and door number two, normal or abnormal. I believe there is something in-between, a door number one and a half, if you will. It occurs in people, as in the lady on NPR. One Medicine predicts it occurs in pets. I call this condition borderline or subclinical hypothyroidism, and I treat it.

In an attempt to address the critics of canine borderline hypothyroid, I set about conducting a double-blind, placebo-controlled study of the effects of treating borderline hypothyroidism in aggressive dogs. To enroll in the study, dogs had to show mild to moderate owner-directed aggression, have one or two clinical signs of hypothyroidism, and have a thyroid hormone levels at the lower end of the normal range. They were treated either with L-thyroxine or identically shaped and colored placebo tablets. The results of this study showed that the frequency of aggressive events was significantly lower in the thyroid hormone–treated group versus the placebo group after six weeks of therapy.

While this sounds fairly conclusive, I must admit there’s a caveat. None of the dogs in the study showed the same dramatic responses that I saw from time to time in the clinic. While the jury might still be out on borderline hypothyroidism, I firmly believe it exists. Clinical evidence, at least, is mounting that dogs can have the borderline syndrome of subclinical hypothyroidism. Because affected dogs show increased anxiety, aggression, and skittishness, and aggression is a leading cause of dogs being put to sleep, it is vital that these avenues of research be pursued.

With hyperthyroidism—elevated thyroid levels, as to the opposite problem of hypothyroidism—there are few controversial issues at all. People suffer from hyperthyroidism, and cats frequently become hyperthyroid when they are elderly. Occasionally, dogs with thyroid tumors become hyperthyroid.

Hyperthyroidism occurs when the thyroid gland is overactive and produces too much thyroid hormone. I always recall the Mark Twain line: “Too much of anything is bad, but too much good whiskey is barely enough.” Well, too much thyroid hormone in the bloodstreams of both people and pets can be disastrous.

Graves’ disease is the most common cause of hyperthyroidism in people. It is caused by the aberrant production of antibodies that mimic a regulatory hormone secreted by the pituitary gland.

In cats, hyperthyroidism is one of the more common conditions of older felines. Ingestion of certain soy-based proteins found in some commercially available canned cat food is a possible cause of hyperthyroidism in cats. Another theory is that it results from the ingestion of flame retardants that are now ubiquitous in home furnishings, including rugs, upholstery, and curtains. These chemicals eventually wear off the furnishings and become incorporated into house dust, which then gets in the cat’s fur, and the cat licks them off when grooming.

Signs of hyperthyroidism, both physical and behavioral, are common across the species, including high blood pressure and rapid heart rate. Behavioral signs include irritability and aggression, excessive reaction to stimuli and increased appetite, paradoxically accompanied by weight loss. Treatment by means of drugs, surgery, or radiation therapy is effective at reversing these signs. I have treated several hyperthyroid cats. One that is most memorable to me is Baby, a cat who lived with me for a period of time toward the end of his life. Baby’s owner had died. I inherited the cat, getting nominated for the job as one of a few people who could have cared for him because of his aggressive behavior.

In his previous home, Baby was hyperkinetic, hypervocal, highly irritable, and extremely aggressive. He also had an increased appetite, was skinny as a rail, and had a disheveled hair coat. I diagnosed him as having hyperthyroidism, and began treatment with a medicine called Tapazole. This twice-a-day drug stops the thyroid gland from making too much thyroid hormone. Even on this treatment, Baby remained somewhat feisty. But many of his other behavioral signs improved.

I recall that getting the treatment into him proved a bit of a chore. My wife, Linda, solved the problem in an ingenious manner. She simply grabbed him, placed him on the corner of my daughter’s bed, and pilled him. Then—and this was the most important step in the process—she fed him. It was not long before he jumped on the bed and waited eagerly for his pill, because it signaled the arrival of food.

I once diagnosed hyperthyroidism in a dog by accident. I was leaving the gym one day when a friend spotted me across a parking lot.

“Hey, Nick, my dog has started to act aggressive to other dogs,” he called out. “I have no idea what to do. Do you have any quick advice?”

As I was about to step into my car at the time, I shouted back the first thing that came to my mind. “Take him to your vet and get his thyroid checked. Let me know what you find.”

A few months later, I met the man’s wife at the same gym. She ran up to me, threw her arms around me and gave me a big hug. “Thank you so very much,” she said. “You hit the nail right on the head. Our dog’s problem was a result of a thyroid issue. If you hadn’t suggested we check his thyroid gland we might never have found out.”

She went on to explain that the dog’s thyroid level had tested out extremely high. He proved to have a thyroid tumor, which was operated upon. Recovered, the dog now was behaving like his old self. Since I was expecting hypothyroidism to be the diagnosis, but had not imagined a tumor could be the cause, this came as quite a shock. I simply said, “Happy I could help.”

The good news is that hyperthyroidism symptoms across the species are pretty much the same. Dogs and cats do not need to have language in order for vets to see what is happening. And the condition improves with the same treatments in both human and nonhuman animals alike.