Introduction to Hypothyroidism in Dogs
The thyroid and parathyroid glands are attached to the trachea or wind pipe about your "adam's apple" is located.
The parathyroid glands produce a hormone that regulates calcium in the body. We are going to ignore the parathyroid glands for this discussion other than to note that they are in the same place in the body as the thyroid glands. I can't remember a single patient in my practice having parathyroid disease.
The thyroid glands produce several major hormones that help regulate:
Growth & development
Cell metabolism by stimulating protein production and oxygen consumption
Calcium regulation (by countering parathyroid hormone).
The abnormalites of the thyroid that we commonly see in practice are over production of thyroid hormones which is called hyperthyroidism (common in old cats) and under production, which is called hypothyroidism (common in middle aged dogs)
This page is about the common disease in dogs called hypothyroidism.
The main cause of the disease is auto immune. For reasons we don't understand yet, the immune system decides to attack and kill the cells of the thyroid gland making them non-functional.
Since thyroid hormones affect just about every cell in the body, we get a wide range of symptoms from not having an adequate level of thyroid.
These symptoms include
Low energy; no pep and dull witted
Poor hair coats and hair loss
Black pigmentation to the skin
Weight Control Issues
Poor disease resistance
Slow heart rate
Inability to keep warm
Chronic ear wax problems
But the disease is often subtle. Your pet may have only a few of the above symptoms.
There must be a genetic factor involved because this disease is more common in some breeds than others. The high risk breeds include Golden Retrievers, Irish Setters, Miniature Schnauzers, Dachshunds, Cockers, Doberman's, and Airedales. And quite common in Shelties.
The disease is rare in the tiny breeds.
And it's a rare problem in young dogs.
Diagnosis is straight forward.
Either you or your veterinarian suspects that hypothyroidism may be a problem because your pet is fat, has excessive shedding, a lousy hair coat, bad skin, or excessive ear or eye discharge.
Or we detect it ... by accident ... more or less while doing a general blood work up
At any rate, we diagnois the problem with blood work using some combination of the following tests:
T4 test: this test is the least expensive and a lot of clinics have the equipment to do this test on site, so this is the most common test done. It's a good screening test. If the result is normal, we can be pretty confident that your pet doesn't have thyroid disease.
If, on the otherhand, the T4 levels are lower than normal, then we have good reason to suspect hypothyroidism ... but the diagnosis is still not certain.
The trouble with this test is that T4 levels (one of the 3 major hormones made by the thyroid gland) normally go up and down (like glucose) during the day.
And thyroid levels are influenced by illness, diabetes, pregnancy, parasitism, diet and certain medications, especially steroids and anti-depressants.
T3 Test: Another screening test that can be run is the baseline T3 test. T3 is another form of thyroid hormone found in the bloodstream. This test can be used as a screening test instead of T4. The T3 test is not as accurate in early cases of hypothyroidism. For these reasons, this test is not often used; and if it is used, it is in combination with the TSH level or TSH stimulation test which are discussed below.
Free T4 by Equilibrium Dialysis: T4 is present in two forms in the body. The "bound" form is attached to proteins in the blood and is unable to enter the cells. The "free" T4 is not attached to proteins, and can readily enter the cells and perform its function. The free T4 is normally present in very small amounts. A special laboratory test - equilibrium dialysis - has been designed that can quite precisely measure free T4.
TSH Level: In the normal animal, the brain controls thyroid hormone levels by secreting thyroid stimulating hormone (TSH) in the bloodstream whenever it detects inadequate thyroid levels. Normally, an increase in TSH triggers the thyroids to make and release more T3, T4, and other hormones (calcitoin) We now have lab tests for measuring TSH.
In a hypothyroid dog, the level of TSH will be elevated because the brain is trying to stimulate the thyroid to produce more hormones. But because the thyroid gland is diseased... not much happens.
So, if the T4 and T3 are low and the TSH is high, we now pretty much have proof that your pet has hypothyroidism and therefore life long treatment would be appropriate.
TSH Stimulation Test: If a dog has a low T4 or T3 level, this test may be performed to confirm a diagnosis of hypothyroidism.
A small amount of Thyroid Stimulating Hormone (TSH) is injected into the vein. After 5-6 hours,we recheck the T4 level. A dog without thyroid disease should now have a high T4 level after the TSH injection. A dog with true hypothyroidism will not have an increase in T4 after the injection. At present, this test is not commonly done in typical vet clinics though ... as informative as it is ... due to expense.
Other Tests: there are other, more sophisticated tests available that specialists use for diagnoising the less common types of thyroid diseases
The treatment is simple and fairly inexpensive. And usually quite successful. Most dogs respond well to thyroxine, available in generic tablet form or slightly more expensive chewable tablets. These are given once or twice daily for life.
Your vet will start your dog off with an appropriate trial dose and then retest your dog's thyroid levels several times ... adjusting the dose as needed ... until the best dose level is found for your pet. Typically, we recommend testing about once a year after that to make sure we are dosing your pet at optimum levels.
This tends to be a rewarding disease to treat in that about 4-6 weeks after initial treatment you can see a big improvement in your dog's skin, energy level, weight and general health.
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