With weight gain and energy loss, this canine glandular disease is especially common in young dogs.
Hypothyroidism is most common in medium to large dogs, and is seen mostly often in animals between the ages of two and six years old.
Consider yourself fortunate if your dog is naturally fit, athletic, high-spirited, quick-witted and blessed with a healthy haircoat. Some dogs aren't like that. They're sluggish, lazy, slow-moving and unresponsive. They have a shabby-looking coat and a tendency to gain weight - even though their owners make sure their daily diet is healthful in terms of both quantity and nutritional value.
In some cases, dogs may be "naturally" fat and lethargic. But if your animal seems to be gaining weight steadily while becoming increasingly lazy and dull-minded, you should consult your veterinarian. A wide variety of disorders can be responsible, one of the more common of which is a glandular disease called hypothyroidism.
Most cases of this frequently observed canine health problem are easily treatable. If untreated, however, its consequences can be lethal.
A Vital Process
Canine hypothyroidism is a condition caused by a malfunctioning thyroid gland - a relatively small, butterflyshaped structure with two lobes, right and left, that are situated in a dog's neck adjacent to the upper end of its trachea, or windpipe. It is a crucial component of an animal's endocrine system, a group of glands located throughout the body that secrete hormones into the bloodstream and thereby regulate many vital bodily processes.
The adrenal glands, for example, secrete hormones that help keep blood pressure and heart function in check and control the body's use of fats, proteins and carbohydrates, while the pancreas produces insulin to regulate blood sugar levels. The thyroid gland's function, on the other hand, is to regulate an animal's overall metabolism - the speed at which all of its bodily processes are carried out.
The thyroid gland accomplishes this by secreting two vitally important hormones, thyroxine and triiodothyromine - commonly known, respectively, as T4 and T3 - that circulate through a dog's system and stimulate receptors in the tissues of the body. When the thyroid is operating properly, a dog's metabolic rate is smoothly maintained. When the gland secretes either too much or too little T3 and T4, however, all of a dog's bodily processes either accelerate or slow down, and the animal can eventually end up in big trouble.
Hypothyroidism, a dysfunction of the thyroid gland that is frequently observed in young to middle-aged dogs, is marked by insufficient hormonal production that occurs when the gland becomes atrophied or, for some reason, disappears altogether. According to Mark Peterson, DVM, head of the endocrinology division at the Animal Medical Center in New York City, the condition is the most common of all canine endocrine diseases. At his clinic, he says, "We see about one case per week."
Hypothyroidism most often results from an immune-system reaction that causes the thyroid gland to become inflamed and then dry up. Cancer, which necessitates the surgical removal of the gland, is another cause of the condition.
In any case, the resulting decline or absence of thyroid hormone production slows down an affected dog's metabolic rate, and this will inevitably lead to the classic clinical signs of the disease: (1) weight gain without a corresponding increase in appetite or food consumption and (2) lethargy often manifested by an unwillingness to exercise and an intolerance of physical activity.
Other frequent signs of the disorder include hair loss (alopecia), which Dr. Peterson notes is usually bilateral and symmetrical; intolerance of cold temperatures; slowed heartbeat; vomiting and diarrhea; and in some instances, an increase in aggressive behavior.
In some cases, canine hypothyroidism will affect a dog's reproductive system. In females, the length of time between estrous cycles may increase; some females, in fact, will stop cycling altogether. In males, the sperm count may decline, the testicles may atrophy, and the sex drive will diminish substantially or altogether disappear.
As Dr. Peterson puts it, "Canine hypothyroidism is a whole-body disease." Although successfully treatable in the great majority of cases, he notes, "the metabolic rate can get so low that the dog may die."
Dogs in whom the condition is diagnosed tend to be young or middleaged. 'They're usually between the ages of two and five or six years old," says Dr. Peterson. "The disease can occur in dogs of any age, however, although a 12- or 13-year-old with hypothyroidism would be pretty unusual."
Although genetic predisposition for the condition is believed to play a role, the extent to which the condition is inherited remains unclear. However, it is clear that the disease is more common in some breeds than in others. According to Dr. Peterson, "Hypo-thyroidism is most common in middle-sized to larger dogs, while tiny toy dogs typically don't get it, although they can."
Among breeds with an apparent increased risk of hypothyroidism are Afghan hounds, Airedales, Labrador retrievers, boxers, Doberman pinschers, golden retrievers, Great Danes, Irish setters, Irish wolfhounds and Newfoundlands. Smaller breeds that also appear to be at higher risk include beagles, dachshunds and Pomeranians.
Fortunately, says Dr. Peterson, hypothyroidism is familiar to veterinarians and relatively easy to diagnose. He recommends the diagnostic procedure for all younger dogs that are gaining weight despite no changes in dietary content or amount of intake while becoming increasingly lethargic. He describes the procedure as follows:
A suspicion of hypothyroidism is based on the dog's history, clinical signs and abnormalities that show up in a physical examination. In most dogs, he notes, the suspicion of hypothyroidism can be easily confirmed by a veterinarian. Standard tests, such as a complete blood count, biochemical profile and urinalysis, will be performed.
In addition, a veterinarian will measure the level of the T4 hormone that is circulating in the patient's blood. "This test may reveal a low level of the hormone," says Dr. Peterson, "which may support a diagnosis of hypothyroidism, but this may be inconclusive, since a lot of different factors can lower the level. Glucocorticoids, for example, are often given to dogs to treat hair loss. A low T4 level in those dogs really wouldn't mean anything.
So we'll also do what's called a free T4 test, which measures the level of hormones in circulating blood that are not attached to proteins. And we may do a thyroid stimulating hormone assay. If the thyroid is failing, the pituitary gland, which makes this hormone, will be working to compensate."
On the basis of the free T4 test and the thyroid stimulating hormone (TSH) assay, says Dr. Peterson, a diagnosis of hypothyroidism can be reliably achieved.
A Simple Treatment
Treatment for the condition is "simple," he says. "All you have to do is replace the missing hormone. So you start right away by giving the dog a synthetic T4 hormone called levothyroxine once or twice a day in pill form, and then you continue doing that for the rest of the dog's life."
The dosage and frequency of administering the synthetic hormone will be determined by a veterinarian and based primarily on the dog's weight. According to Dr. Peterson, a daily supply of levothyroxine will typically cost the owner between fifty cents and a dollar.
The prognosis for an animal that is taking its pills as prescribed is "great," says Dr. Peterson. "We'll check the dog every two or three weeks at first and every few months thereafter," he notes, "in order to monitor its improvement and adjust the dose up or down as needed to make sure it's right."
Although canine hypothyroidism cannot be prevented, many of the signs associated with the condition begin to resolve within a matter of weeks. Owners are advised, however, to keep close watch over their dogs in the early stages of treatment with the synthetic hormone, since overdoses of levothyroxine may produce symptoms of hyperthyroidism: an excess, rather than an insufficient amount, of thyroid hormone in the system.
These symptoms would be likely to include dramatic and clearly apparent weight loss and episodes of hyperactivity - and you don't want that, either.