Medicine: Hypothyroidism

The thyroid glands are located in the neck on either side of the windpipe. These very important glands control a lot of body functions by secreting thyroid hormone, which a special protein in the blood which regulates many organs. It controls a lot of aspects of metabolism. Too much thyroid hormone speeds up metabolism and causes many problems. This is called hyperthyroidism and is common in cats and very rare in dogs. If the thyroid hormone levels are low, then metabolism tends to slow down. This condition is called hypothyroidism. It is the most common hormonal disease in dogs.

Hypothyroidism has three main causes. One cause is by over-treatment of hyperthyroidism and will be discussed next month. Another cause is atrophy or reduction in size of the thyroid gland. In this degenerative process, active thyroid tissue is replaced by fat. The most common cause believed to be caused by an immune attack on the thyroid gland. This is called lymphocytic thyroiditis and basically the immune system believes that the thyroid hormone tissue is foreign and attacks it. Although unproven it is suspected that hypothyroidism has a genetic basis. Unfortunately, too few breed studies have been done to determine actual percentages of breeds which are affected. Breeds in which we see hypothyroidism commonly are:

Afghan Hound
Airedale Terrier
Alaskan Malamute
American Cocker Spaniel
Beagle
Boxer
Borzoi
Chinese Shar Pai
Chow Chow
Dachshund
Dalmatian
Doberman Pinscher
English Bulldog
Golden Retriever
Great Dane
Irish Setter
Irish Wolfhound
Labrador Retriever
Miniature Schnauzer
Newfoundland
Pomeranian
Poodle
Shetland Sheepdog

Certainly we have seen hypothyroidism in other breeds as well, but these are the most common.

What are the symptoms of thyroid disease? There are many, and some can be subtle. Dogs generally don’t show symptoms until late in the course of the disease and the symptoms are hard to spot, because they are slowly progressive in onset. Some dogs show all symptoms, some very few at time of diagnosis.

• weight gain
• lethargy or decreased activity
• hair loss, bilateral (even on both sides) of the body
• poor hair re-growth after grooming
• cold intolerance/seeking warmth
  
• scaling/crusting/odor of the skin/dry coat
  
• recurrent itching/allergies
  
• recurrent ear infections
  
• darkening of the skin
  
• thickening of skin on the face, creating a “tragic expression”
  
• abnormal nerve function causing non-painful lameness
  
• loss of libido/infertility in males
  
• lack of heat periods/infertility/abortion in females
  
• decreased tear production (dry eye)
  
• fat deposits on the cornea (clear part of the eye)

How is it hypothyroidism diagnosed? Certainly, history of symptoms increases suspicion. An elevated blood cholesterol is also increases our suspicion. On all dogs over age seven we like to run thyroid checks to determine thyroid function.

There are very specialized blood tests to determine thyroid function.

• T4: The most common and least expensive test is the T4 test. It is a great screening test, in that it has few false negatives (dogs which have the disease but test results are normal.) Unfortunately, a low result does not mean that your dog has the disease. Many dogs fall outside the normal range. Also, sick dogs get a decrease in their circulating T4 levels. This is called sick euthyroid syndrome. It is for this reason that T4 tests are not run by themselves on sick dogs.
  
  
• Free T4: Contrary to owner wishes, this test is not “free”. Instead it measures the thyroid hormone not bound by proteins. It is more accurate than the T4. It can be normal in early stages of the disease. It is more expensive than the T4, so we only run this test if the T4 is low or we are highly suspicious of hypothyroidism. Only the free T4 run by a process called Equilibrium Dialysis (ED) is accurate. If both the T4 and free T4 are low, then we will treat. If the tests are equivocal, then we run more tests.
  
  
• cTSH: This is a hormone made in a gland near the brain called the pituitary gland. It increases activity of the thyroid glands. Thyroid hormone suppresses this hormone, so there is a great feedback mechanism. If this hormone is high on the blood test, it means that too little feedback is occurring, which means too little thyroid hormone is in the blood (hypothyroidism.) It is very accurate, even early in the disease. It is also more expensive than other tests.
  
  
• Thyroid antibodies: This tests checks for antibodies against the thyroid hormone precursors. It is great test for lymphocytic thyroiditis. It does not detect atrophy of the gland. It is expensive.
  
  
• Other tests: There are other blood tests for thyroid disease. The results from them are generally equivocal and not performed any longer as the Free T4 and cTSH tests are easy to perform and highly accurate.

What does it mean if your dog is hypothyroid? Is it fatal? Hypothyroidism is not by itself often fatal, but it can lead to metabolic dysfunction and a suppressed immune system, which can lead to other diseases. Luckily, treatment is easy. Hypothyroid dogs need to be supplemented with a synthetic thyroid supplement. This needs to be given lifelong, as the thyroid function never returns to normal. The supplement, called levothyroxine, is given twice daily. There are many trade names for this drug (Soloxine, Synthroid, Thyroxine, Thyrotabs, etc.) but the active ingredient is the same. After a month on treatment, about 4-6 hours after the morning pill is given, we draw a thyroid test. If it is in the normal range, we continue the dose and re-test yearly. If it is abnormal, we adjust the dose and re-test again in about a month.

My dog’s was diagnosed with thyroid disease and his thyroid results are always too high when on medication. This could be because the dosage is just too high. Also, it could be an incorrect diagnosis. Unfortunately, because some veterinarians aren’t using these newer tests, a lot of dogs are on a thyroid supplement unnecessarily. If you think your pet might be exhibiting weight loss and hyperactivity when on thyroid supplement, you may want a thyroid test checked to make sure the dose isn’t too high.

You said that thyroid disease could be genetic. How can I be sure that the puppy I purchased won’t have the disease? Unfortunately, there is no way to be sure. But there is a way to make sure that the parents do not have the disease. The Orthopedic Foundation for Animals (OFA), the same group that has a registry to check for hip dysplasia, also has a registry for hypothyroidism. T4, free T4, cTSH, and thyroid autoantibody tests are performed, and a certificate and registry are available for dogs tested at 12 months of age. The tests are redone at 2, 3, 4, 6, and 8 years of age. I certainly would not purchase a high-risk breed without this testing or certificate. Breeders should think twice before breeding a hypothyroid dog.