Historically a disease of “middle-aged” dogs, hypothyroidism occurs when their thyroid glands no longer produce enough thyroid hormones.  

The two main causes of this condition are lymphocytic thyroiditis and idiopathic thyroid gland atrophy. The former is the most common and is thought to be an immune-mediated disease whereas in the latter, normal thyroid tissue is replaced by fat tissue. Accounting for more than 95% of canine hypothyroidism, the other five percent are the result of rare diseases, including cancer of the thyroid gland itself.

 If a dog is suffering from hypothyroidism, the rate of his metabolism slows down, thereby affecting most of his vital organs. As a result, some of the symptoms he’s most likely to exhibit include:

• Weight gain without an increased appetite

• Lethargy and a lack of desire to exercise

• A slow heart rate

• High blood cholesterol

• Intolerance to the cold

• An increased susceptibility to and the occurrence of ear and skin infections

• An increase in the dark pigmentation of his skin

• Dry and dull hair with excessive shedding

• A very thin to a nearly bald coat

• An inability to re-grow his hair after being clipped or shaved

Some dogs may also exhibit:

• Keratoconjunctivitis sicca (KCS) or dry eye due to a lack of proper tear production

• Fat deposits in the corneas of his eyes

• A thickening of his facial skin, giving him a woebegone expression

• A head tilt, foot dragging and/or a lack of coordination

• A lack of heat periods, infertility and miscarriages in intact females

• A loss of libido and infertility in intact males

To properly diagnose hypothyroidism, your vet will order a group of blood tests that will help determine the levels of your dog’s thyroid hormones. The most common test is a T4 test -- a measurement of the blood levels of total thyroxine, the main hormone secreted by the thyroid glands. Usually run concurrently are TSH (thyroid-stimulating hormone) and free T4 tests. They provide especially essential information since T4 levels can not only vary but may be low because of an illness totally unrelated to a thyroid problem.

While hypothyroidism may not be curable, the condition is treatable. It involves administering an oral synthetic thyroid hormone replacement to your dog for the rest of his life. The drug most commonly prescribed is levothyroxine (brand names: Thyro-Tabs Canine®, Synthroid®, Levothroid®, Levoxyl®, Unithroid®, Levo-T®, Eltroxin® and PMS-Levothyroxine®).

Whether in tablet or capsule form or compounded into a liquid, every dog is started on a standard dose of the thyroid replacement hormone based on his weight. This medication can be given with or without food but it should be given the same way at the same time every day. For most dogs, an improvement in their attitude and activity level can usually be seen within one or two weeks while such clinical abnormalities as flaky skin and hair loss may take a few weeks to a few months to resolve.

Your vet will subsequently re-check your dog’s thyroid hormone levels by ordering a series of blood panels to ensure that the drug is working. But because his tolerance to the drug may change over time, the dose may need to be adjusted periodically.

Not only is it vital, then, to have your dog’s hormone levels tested every six months, it’s equally important to monitor his condition at home and alert your vet to any and all changes you may see. But take heart: dogs treated for hypothyroidism have an excellent prognosis and their life expectancy is normal.