Many veterinarians are unaware of how to properly test and diagnose their patients in order to identify the early stages of this disease process, prior to the development of classical clinical signs. Since the thyroid gland regulates metabolism of all bodily cellular functions, reduced thyroid function can produce a wide range of clinical manifestations, which mimic those of other conditions.

Most pet owners and even many dog breeders do not know how to spot the clinical or behavioral signs of early canine thyroid disease. This confusion sets up a “perfect storm” of misunderstanding, misdiagnosis and mistreatment of canine thyroid disorders.

The heritable nature of thyroiditis poses significant implications for breeding stock, so accurate diagnosis is important for genetic counselling and provides the clinical options for prompt intervention and case management.

Behavioral issues

The principal reason for pet euthanasia stems not from disease, but undesirable behavior. While abnormal behavior can have a variety of medical causes, it also can reflect underlying problems of a psychological nature.

An association between behavioral and psychological changes and thyroid dysfunction has been recognized in humans since the 19th century. In a recent study, 66% of children with attention deficit-hyperactivity disorder (ADHD) were found to be hypothyroid, and supplementing with thyroxine was largely curative. Furthermore, an association has recently been established between aberrant behavior and thyroid dysfunction in the dog, and has also been noticed in cats with hyperthyroidism. Typical clinical signs include unprovoked aggression towards other animals and/or people, sudden onset of seizure disorder in adulthood, disorientation, moodiness, erratic temperament, periods of hyperactivity, hypoattentiveness, depression, fearfulness and phobias, anxiety, submissiveness, passivity, compulsiveness and irritability. After episodes, most animals appear to come out of a trance-like state and are unaware of their bizarre behavior.

The mechanism whereby diminished thyroid function affects behavior is unclear. Hypothyroid patients have reduced cortisol clearance, as well as suppressed TSH output and lowered production of thyroid hormones. Constantly elevated levels of circulating cortisol mimic the condition of an animal in a constant state of stress. In people, and seemingly in dogs, mental function is impaired and the animal is likely to respond to stress in a stereotypical rather than reasoned fashion.

Investigators in recent years have noted the sudden onset of behavioral changes in dogs around the time of puberty or as young adults. Many of these dogs belong to a certain group of breeds or families susceptible to a variety of immune problems and allergies — e.g. golden retriever, Akita, Rottweiler, Doberman pinscher, English springer spaniel, Shetland sheepdog, and German shepherd. The clinical signs in these animals, before they show the sudden onset of behavioral aggression, can include minor problems such as inattentiveness, fearfulness, seasonal allergies, skin and coat disorders and intense itching. These may be early subtle signs of thyroid dysfunction, with no other typical signs of thyroid disease being manifested.

The typical history starts out with a well-mannered and sweet-natured puppy or young adult dog. The animal is outgoing, attends training classes for obedience, working or dog show events, and came from a reputable breeder whose kennel has no prior history of producing animals with behavioral problems. At the onset of puberty or thereafter, however, sudden changes in personality are observed. Typical signs can be incessant whining, nervousness, schizoid behavior, fear in the presence of strangers, hyperventilating and undue sweating, disorientation and inattentiveness. This can progress to sudden unprovoked aggression in unfamiliar situations with animals, people and especially children.

Another group of dogs shows sudden onset seizures or seizure-like disorders that can occur at any time from puberty to mid-life. These dogs appear perfectly healthy, have normal hair coats and energy, but will suddenly seizure for no apparent reason. The seizures are often spaced several weeks to months apart, may coincide with the full moon, and can appear in brief clusters. In some cases, the animals become aggressive and attack those around them shortly before or after having one of the seizures.

In dogs with aberrant aggression, a large collaborative study between our group and Dr. Dodman and colleagues at Tufts University School of Veterinary Medicine has shown a favorable response to thyroid replacement therapy within the first week of treatment, whereas it took about three weeks to correct their metabolic deficit. A dramatic reversal of behavior with a resumption of previous problems has occurred in some cases if only a single dose is missed. A similar pattern of aggression responsive to thyroid replacement has been reported in a horse.

Our ongoing study now includes over 2,500 cases of dogs presented to veterinary clinics for aberrant behavior. The first 499 cases have been analyzed independently by a neural network correlative statistical program. Results showed a significant relationship between thyroid dysfunction and seizure disorder, and thyroid dysfunction and dog-to-human aggression.

Collectively, these findings confirm the importance of including a complete thyroid antibody profile as part of the laboratory and clinical work up of any behavioral case.

Two case studies

1. Casey, a 3.5-year-old intact male golden retriever, suddenly began showing signs of abnormal behavior. He became vocal, reactive to unknown dogs, easily excited, agitated and restless. Over the period of a year, Casey became progressively more aggressive, going so far as to bite household members several times.

Casey’s veterinarian performed blood tests that confirmed he suffered from autoimmune thyroiditis. He was placed on thyroxine at a dose of 0.35 mg administered once daily. However, this small dose was clearly inadequate for his size. In addition, the medication was only being administered once daily, when its half-life is 12 to 16 hours. When Casey showed no signs of improvement after two months on therapy, the veterinarian stopped the thyroxine for two months and retested him. During that time, Casey’s aggressive behavior escalated. Upon retest a couple of months later, his blood work displayed the classical signs for hypothyroidism.

Fortunately, Casey was placed on thyroxine once more – now at a correct dose and administered twice daily at about 12-hour intervals. Casey showed almost immediate improvement, and after one year, all his blood hormone levels normalized. He was also neutered, so as not to pass along the inherited condition to any offspring.

2. Terry, a 4.5-year-old intact male bearded collie, experienced a seizure at three years of age. He recovered quickly and nothing abnormal was found on physical examination or routine blood and urine tests. Terry’s behavior, however, started to become noticeably more unpredictable. He would curl his lip, growl threateningly and bark when he didn’t want to do as told. This behavior could be triggered during grooming and bathing or when he refused to come when called. He was excitable and hyperactive, but had never bitten anyone. He otherwise appeared healthy, and had a very good diet and plenty of exercise. He had tried to chase and attack sheep at herding trials, but was very friendly with cats, people and small children.

Terry’s guardians took him back to the vet clinic and requested a full thyroid profile. The thyroid tests indicated a low level of three thyroid hormones, although there was no elevation of thyroid autoantibodies. Terry was placed on the appropriate dose of thyroxine for his weight, to be given BID. After just one week of therapy, a miracle seemed to happen. His behavior steadily improved, he was much calmer and no longer anxious, and his aggressive tendencies had diminished. A retesting of his thyroid profile after eight weeks showed an improvement in thyroid levels to match the resolution of his behavioral issues.


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Dr. Jean Dodds received her veterinary degree in 1964 from the Ontario Veterinary College. In 1986, she established Hemopet, the first non-profit national blood bank program for animals. Today, Hemopet also runs Hemolife, an international veterinary specialty diagnostics service. Dr. Dodds has been a member of many committees on hematology, animal models of human disease and veterinary medicine. She received the Holistic Veterinarian of the Year Award from the AHVMA in 1994, has served two terms on the AHVMA’s Board of Directors, chairs their Communications Committee, and currently serves on the Board of the AHVMF, as well as its Research Grant and Editorial Committees.