Hypothyroidism occurs when the thyroid gland doesn’t produce and secrete enough thyroid hormones. The thyroid gland is located in the mid-neck region near the voice box (larynx). In dogs, the thyroid gland is made up of two separate lobes that lie on either side of the windpipe (trachea). Thyroid hormones play a big role in metabolism and affect the function of many parts of the body.

Hypothyroidism is a common hormone imbalance in dogs and is usually caused by inflammation or shrinkage of the thyroid gland. This progressive destruction of the gland leads to diminished thyroid function and insufficient levels of thyroid hormones. Hypothyroidism is typically seen in middle-aged to older dogs and occurs more commonly in medium to large breed dogs.

Because thyroid hormones affect virtually every cell in the body, it’s not surprising that dogs with hypothyroidism show signs in multiple body systems. The destruction of the thyroid gland usually progresses slowly, so signs are often gradual and easy to miss at first. Common findings in dogs with hypothyroidism include:

• Hair loss or thinning of the fur—this occurs mainly over the dog’s trunk, sparing the head and legs. Often, the hair loss starts on the tail leading to a “rat tail” appearance. A bald area around the collar is also common. The skin is usually not itchy or red unless there’s also a skin infection;
• Skin infection—the skin is often scaly and smelly due to an excessively oily coat;
• Dull, dry hair coat—the outer hairs can be brittle and break easily, leaving a short, softer undercoat sometimes described as a puppy-like coat;
• Skin thickening and darkening—this occurs especially in areas of friction, such as the armpit or groin. Often, the skin around the face and head thickens, leading to more skin folds;
• Ear infection—the ears may be red, painful, and smelly;
• Weight gain;
• Reduced activity level—the dog may be listless, tire easily, and sleep a lot;
• Inability to tolerate cold—the dog will seek out warm places;
• Anemia (low level of red blood cells); and
• High cholesterol.

Based on your dog’s signs, your veterinarian may run several blood tests to assess your dog’s thyroid function.

Dogs with illnesses besides thyroid disease or that are on certain drugs may have low thyroid hormone levels but not have hypothyroidism. Drugs that can reduce thyroid hormone levels include:

• Anti-seizure medications, like phenobarbital;
• Prednisone or other steroids;
• Some non-steroidal anti-inflammatory drugs (NSAIDs);
• A heart medication called propranolol;
• A behavior medication called clomipramine; and
• The sulfa class of antibiotics.

Sighthounds, such as Greyhounds, Irish Wolfhounds, and Whippets, normally have much lower thyroid hormone levels than other breeds.

Your veterinarian will consider the results of your dog’s blood tests in light of the whole picture. In some cases, a diagnosis of hypothyroidism may be difficult to confirm and require repeated testing.

There is no cure for hypothyroidism. Dogs must be treated for life with thyroid hormone replacement therapy. Two drugs are FDA-approved for replacement therapy for diminished thyroid function in dogs.

• THYRO-TABS CANINE (levothyroxine sodium tablets)
• ThyroKare™ (levothyroxine sodium tablets)

Both FDA-approved products are prescription drugs containing levothyroxine sodium as the active ingredient. The tablets are available in different strengths. Your veterinarian will carefully calculate the dose depending on your dog’s weight. THYRO-TABS CANINE are given by mouth every 12 or 24 hours and ThyroKare™ tablets are given every 12 hours, as prescribed by your veterinarian.

The amount of drug absorbed by a dog’s body depends on whether it is given with or without food. To minimize variation in absorption, you should consistently give the drug to your dog either with or without food.

You will typically see improvement in your dog’s activity level within a few weeks of starting thyroid hormone replacement therapy. However, hair regrowth takes longer, typically at least 4 months. Skin and ear problems also may take longer to clear up and require additional medications.

Yes. Your veterinarian will monitor your dog’s thyroid hormone levels after starting thyroid hormone replacement therapy to make sure the dose is adequate. If the dose is too low, your dog will not be adequately treated and signs will continue. If the dose is too high, your dog may become restless, nervous, drink and urinate more than normal, and lose weight despite having an increased appetite (see section below on thyrotoxicosis).

After starting thyroid hormone replacement therapy, your dog’s thyroid hormone levels should be initially checked every 4 to 8 weeks. Based on these results, your veterinarian may adjust the dose until your dog is adequately maintained. After that, thyroid hormone levels should be monitored periodically in case your dog’s requirement for thyroid hormone replacement therapy changes over time. 

Dogs with underlying heart disease that are diagnosed with hypothyroidism should be closely monitored by their veterinarians after first starting thyroid hormone replacement therapy. The dose of heart medication or levothyroxine sodium may need to be adjusted depending on the dog’s response to treatment.

Reported side effects of levothyroxine sodium include:

• Not eating;
• Itchiness;
• Skin problems, such as red, inflamed skin;
• Decreased activity level;
• Vomiting;
• Diarrhea;
• Increased drinking and urination; and
• Increased activity level.

FDA encourages you to call your veterinarian if you think your dog is having a side effect from levothyroxine sodium. A side effect associated with a drug is called an adverse event. Adverse events also include a lack of effectiveness (the drug doesn’t do what it’s supposed to do) and reactions in people who handle the drug. Call your healthcare provider if you have a reaction to levothyroxine sodium.

FDA also encourages you to work with your veterinarian to report any adverse event - in either pets or people - associated with levothyroxine sodium or any product defect such as defective packaging or the drug is off-color. How to Report Animal Drug and Device Side Effects and Product Problems.

Remember to keep the drug in a secure location out of reach of children, dogs, cats, and other animals to prevent accidental ingestion or overdose.

Sometimes, a condition called thyrotoxicosis may develop dogs receiving levothyroxine sodium if:

• The dose is too high;
• The dog has an underlying illness that affects the body’s ability to break down the drug; or
• The dog is very sensitive to the drug.

Signs of thyrotoxicosis include panting, nervousness, hyperactivity, a rapid pulse, increased drinking and urination, vomiting, diarrhea, and weight loss despite having an increased appetite. Call your veterinarian if you see these signs in your dog.

Levothyroxine sodium should not be used in dogs with thyrotoxicosis or in dogs with uncorrected adrenal insufficiency. (Adrenal insufficiency—also called hypoadrenocorticism or Addison’s disease—occurs when the adrenal glands, located just atop the kidneys, don’t secrete adequate amounts of certain hormones that, in a healthy dog, regulate the body’s reaction to stress and the balance of the electrolytes sodium and potassium.)

The safety of levothyroxine sodium has not been evaluated in breeding, pregnant, or nursing dogs. In pregnant women and rodents, too much exposure to thyroid hormones is associated with defects in the thyroid gland of their offspring.

Knowing a drug is safe, effective, and high-quality is the benefit of FDA-approval. During the approval process for an animal drug, the agency evaluates information submitted by the drug company to make sure the drug is safe and effective for its intended use and that the drug is properly manufactured and properly labeled.

FDA-approved levothyroxine sodium products have been shown to be safe and effective in dogs when used according to the directions on the label. The drugs are properly manufactured and their labels provide dosing and safety information specific to dogs.

FDA has serious concerns about unapproved animal drugs, including unapproved levothyroxine sodium products for dogs. Unapproved animal drugs have not been reviewed by FDA and may not meet the agency’s strict standards for safety and effectiveness. They also may not be properly manufactured or properly labeled. FDA advises veterinarians and dog owners to avoid using unapproved levothyroxine products.

• Hypothyroidism in Dogs, Washington State University, College of Veterinary Medicine, Pet Health Topics
• Hypothyroidism is the Most Common Hormone Imbalance of Dogs, The Pet Health Library, VeterinaryPartner.com