Hypothyroidism: An Overview

The 13 year-old dog wheezed as she entered the veterinarian's office. Barely able to walk, with a persistent cough and breathing difficulties from the accumulation of fluid in her lungs, she was in bad shape. Her hair coat was thin all over, and she had soft, spongy fat over her chest and abdomen, while her legs were impossibly thin from muscle wasting. Her breath had a sweet smell, sometimes characteristic of dogs with kidney problems. "What can it be?" the rescue worker asked. "Cushing's? A heart problem?" Many tests were run; blood tests to check liver and heart function, urine tests for kidney problems. Many diseases common in very old dogs were ruled out.

The 6 year-old Chessie bitch had been in perfect health. Overnight, she was losing bladder control. Her owner awoke to find the bed soaked with her dog's urine. "Spay incontinence, or maybe kidney failure from Lyme disease." was the vet's dire prediction. Again, tests were run, this time consistent with diseases that healthy, active working dogs might get. Screens for Lyme and other tick-borne illnesses, hormone checks, and tests for other infections were done. All came up negative.

What did these dogs have in common? Both had hypothyroidism, a condition where their thyroid hormone levels were too low. In the older dog, the condition had existed for years, undetected. The previous owner just chalked her gradually declining health up to "old age", and never investigated. The younger dog had no symptoms other than the bladder problem, and in fact, her initial thyroid screens came back normal, with only the T4 value registering in the lower end of the normal range.

The thyroid gland lies across the throat, just under the skin of the neck.

It creates two hormones, T3 (triiodothyronine) and T4 (thyroxine) that are used by literally every cell in the body to regulate cell metabolism. Most thyroid hormone becomes bound to protein molecules circulating in the blood. It is the relatively small percentage of unbound hormone (called free T4 and free T3) that is actually used by the cells. When the thyroid gland does not produce adequate thyroid hormone or something happens to affect the amount of free hormone available to the cells, the cells go without, and do not function correctly. If the thyroid insufficiency goes on for too long, many of the body's systems begin to fail. In the case of the 13 year-old rescue dog, long-term hypothyroidism led to many other health issues.

Hypothyroidism is the most commonly diagnosed endocrine (glandular) condition in dogs. Because it can mimic many other conditions, it often times is not the first thing that veterinarians will check for when a dog comes in with symptoms. Because virtually every system in the dog's body is affected by thyroid hormone, dogs with low thyroid often have very different symptoms from each other. This makes diagnosing hypothyroidism very difficult, as it sometimes seems no two dogs have the same symptoms.

While hypothyroidism is often underdiagnosed, at the same time it is often over-diagnopsed. Many vets will reach for thyroid supplements automatically without testing, because they are becoming more aware of how common the condition is in dogs. This is a poor practice, however, because while a dog may have hypothyroidism, it may also have another condition or conditions that the thyroid problem is contributing to. Simply adjusting the dog's thyroid hormones will not resolve any other underlying conditions. In addition, supplementing without testing first may mean treating the thyroid when there is nothing really wrong with it. Meanwhile, the true, underlying condition goes undiagnosed and untreated.

Because thyroid hormone affects every cell of the body, the symptoms of hypothyroidism can be varied. Here are some symptoms and conditions that can be caused by hypothyroidism:

• Weakenss or stiffness caused by muscle wasting
• Seizures
• Aggression
• Head tilt
• Knuckling or dragging feet
• Drooping eyelids
• Tragic experssion (caused by changes in fat distribution under the skin of the face)
• Lethargy
• Mental dullness
• Exercise intolerance
• Weight gain
• "Spongy" fat over the ribs and abdomanal area
• Cold intolerance
• Chronic skin, ear, or urinary tract infections
• Dry, scaly skin and dandruff
• Coarse, dull coat
• Bilateral symmetrical hair loss
• Rat tail
• "Puppy coat" - thick, soft, plush coat that never sheds
• Seborrhea or greasy skin
• Chronic offensive odor
• Infertility in either sex
• Lack of libido
• Testicular atrophy
• Hypospermia (low sperm count)
• Aspermia (no sperm produced)
• Prolonged intraestrus interval
• Absence of heat cycles
• Silent heats
• False pregnancy
• Weak, dying, or stillborn pups
• Slow heart rate
• Cardiac arrythmias
• Cardiomyopathy
• Bleeding
• Bone marrow failure
• Low red blood cell count
• Low white blood cells
• Low platelet count
• Corneal ulceration
• Dry eye
• Loss of sense of smell

As we can see, the list of symptoms is long. Hypothyroidism is the "great pretender", and can look like many other diseases, such as Cushing's, diabetes, cardiomyopathy, or epilepsy. That's why it is so important to check a dog's thyroid function if it shows signs of other diseases. While the dog may actually have those diseases, if it is also hypothyroid, the thyroid insufficiency may be contributing to the dog's overall poor health. Or, the dog's poor condition may be entirely caused by low thyroid, and the dog may have none of these other conditions.

There are two broad types of hypothyroidism: primary hypothyroidism, where the thyroid gland itself is not functioning correctly, and secondary hypothyroidism, where the thyroid gland is functioning correctly, but some other process is affecting the body's ability to use the thyroid hormone produced. Both primary and secondary hypothyroidism are slow-developing diseases. It takes a long time for the processes that affect thyroid hormone, whether primary or secondary in nature, to create noticeable changes in the body. In primary hypothyroidism, it is believed that over 60% of the gland must be destroyed before symptoms begin to appear. In secondary hypothyroidism, there are several mechanisms that the body uses to compensate for the inability to utilize circulating thyroid hormone. The changes are very subtle at first; it often times is not until a dog reaches middle age that thyroid insufficiency is discovered.

The most common form of primary hypothyroidism in dogs is caused by an immune mediated process, called lymphocytic thyroiditis. Lymphocytic thyroiditis is believed to be responsible for over half of all cases of primary hypothyroidism. The dog's own immune system begins slowly destroying the thyroid gland. This process can happen fairly slowly, and as long as there is enough healthy thyroid gland tissue, the gland will continue to make hormone. It is not until most of the gland is destroyed that the amount of hormone available becomes too low to keep the body's cells functioning well.

Most other cases of primary hypothyroidism are what is known as idiopathic thyroidis. The dog's thyroid gland begins shrinking, but the cause is unknown. Studies recently completed have negated two of the more popular beliefs (particularly on the internet) regarding causes of hypothyroidism, vaccines, and use of choke chains. Studies conducted specifically to look at the relationship between vaccination and hypothyroidism have shown no relationship between vaccination with polyvalent vaccines and hypothyroidism (Hogenash et al., 1999 Advances in Veterinary Medicine 41:733-747). In other words, vaccinating dogs neither causes nor triggers hypothyroidism, even in animals with a known familial predisposition toward immune-mediated hypothyroidism. The other study, looking at hypothyroidism and behavior (particularly aggression) is ongoing, but has already ruled out external physical damage to the thyroid gland as a cause of hypothyroidism. Part of this study, being conducted in the United Kingdom, was specifically designed to look at whether the use of choke or other restricting collars could damage the thyroid gland and cause hypothyroidism. Fortunately, both with vaccines and choke colars, neither was found to cause hypothyroidism.

In a few very rare cases, hypothyroidism is caused by a pituitary problem. The pituitary gland makes a hormone that is responsible for stimulating the thyroid gland to create more thyroid hormone. When the amount of thyroid hormone in the dog's blood drops below a certain level, this signals the pituitary gland to create more thyroid stimulating hormone (TSH), which in turn signals the thyroid gland to create more thyroid hormone. If there is something wrong with the pituitary gland, it will not function properly in this biofeedback mechanism. Interpretation of thyroid test results will determine which process is responsible for a dog's low thyroid values.

Some other causes involve what is known as sick euthyroid, where a dog's thyroid gland is healthy and functioning normally, yet the thyroid hormone is not being used by the cells because of some other disease process in the body. There are a number of other reasons for a dog to not be able to use thyroid hormone. Some of the more common causes include long-standing infection, pregnancy, or use of certain medications. These can sometimes affect the binding of thyroid hormone to proteins, causing there to be less free (unbound) thyroid hormone available to the cells.

As we can see, there are many possible causes for a dog's thyroid hormone levels to drop. Several tests exist to determine which cause or causes lie behind a particular dog's hypothyroidism. Depending on the outcome of these tests, effective treatment is available, and dogs can live normal, healthy lives when treated with the correct dosage of supplemental thyroid hormone.

(c)2002 by Lisa Van Loo, revised (c)2008

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