Hypothyroidism in Dogs

What is Hypothyroidism in Dogs?

Hypothyroidism is a common endocrine disease in dogs that results in decreased production of thyroid hormones. Thyroid hormones are produced by the thyroid glands, which are located on either side of the neck near the throat. These hormones serve an important role in metabolism. When the glands are not producing enough hormones, the dog’s body functions slow down. 

Dogs with hypothyroidism usually have either inflammation of the thyroid glands or degeneration (deterioration) of the glands. Fortunately, thyroid tumors are fairly uncommon in dogs. 

When it does occur, hypothyroidism is most common in middle-aged dogs, with medium-to-large breed dogs being more commonly affected. Golden Retrievers, Dobermans, Dachshunds, Miniature Schnauzers, Cocker Spaniels, and Irish Setters are among the breeds more predisposed.  

Symptoms of Hypothyroidism in Dogs

Dogs with hypothyroidism often show one or more of the following clinical signs: 

• Weight gain: This often occurs without an increase in appetite. Many pet parents note that their dog seems to be gaining lots of weight even though they don’t eat that much food. 

• Lethargy and laziness: Your dog may prefer to sleep and lie around all day rather than run and play. 

• Heat-seeking behavior: Because of their low thyroid hormone and resulting low metabolism, dogs with hypothyroidism constantly run a little cold. Your dog may prefer to lie near the fireplace or on the heat vent to try and stay warm. 

• Chronic skin and ear infections: While allergies are relatively common in dogs, chronic skin and ear infections may be a sign of an underlying issue with hypothyroidism. 

• Dry and brittle hair with a thinning hair coat: Sometimes a dog with hypothyroidism will lose hair from their back on either side. They may also lose hair from their tail, giving it a rat tail type appearance. 

• Increased pigmentation of the skin 

• Inability to regrow hair after it’s been shaved 

There are other, less common, signs of hypothyroidism that some dogs develop. Dogs may have reproductive problems, or develop issues with their nervous systems, including nerve pain or dragging their hind legs.  

Dogs with low thyroid may have small, white fat deposits on the surface of their eyes or end up with an eye condition called dry eye, where they don’t produce enough tears. Some dogs even have thickening of the facial skin so the muscles of the face droop.  

Causes of Hypothyroidism in Dogs

The two most common causes of hypothyroidism in dogs are inflammation of the thyroid gland (lymphocytic thyroiditis) and degeneration of the thyroid gland (idiopathic thyroid gland atrophy).  

While we are not sure why these two conditions spontaneously occur in some dogs, we know there is a genetic predisposition. Another rare cause of hypothyroidism in dogs includes cancer. Fortunately, these other causes are only responsible for a small percentage of hypothyroid patients.  

Majority of patients with hypothyroidism have either inflammation of their thyroid gland or degeneration of their thyroid gland, and both conditions can be managed with medication.  

How Veterinarians Diagnose Hypothyroidism in Dogs

Your veterinarian will start with a physical examination of your dog, including a thorough medical history.  It’s important to share any unusual behaviors you’ve noted in your dog and be sure to include a time frame of when you first noticed these behaviors or physical changes.  

Your vet may want to run some basic bloodwork and a urinalysis to establish your dog’s overall health. If your dog has changes in their skin, your vet may want to do skin scrapes or smears (sample collected by either gently scraping the surface of the skin with a scalpel or by pressing a microscope slide to the skin) to look under a microscope and rule out any secondary skin infections.  

Hypothyroidism is diagnosed with a blood test. Your veterinarian will want to draw blood from your dog to either test in their clinic or send off to an external laboratory for testing.  

Most commonly, this disease is diagnosed by running a screening test called a total thyroxine level (Total T4, or TT4). This test determines your dog’s main thyroid hormone level. If it is low, and your dog has clinical signs of hypothyroidism, this is suggestive of a diagnosis.  

Many veterinarians will then run additional blood tests, either a free T4 level or a full thyroid panel, to confirm the diagnosis of hypothyroidism. Sometimes dogs can have a low total T4 but not necessarily have hypothyroidism. Occasionally, a dog can have a total T4 that is at the low end of the normal range but still have hypothyroidism. These confirmatory tests can be especially helpful in such cases.  

Treatment of Hypothyroidism in Dogs

Hypothyroidism is treated with an oral medication called levothyroxine. This medication is a synthetic form of the thyroid hormone your dog is missing. It is important to note that hypothyroidism is treatable but not curable.  

Your dog will need to stay on their thyroid replacement hormone for life. This pill comes in several different strengths, so your veterinarian will select the appropriate dose for your dog based on weight. They will likely want to re-check bloodwork in one month to ensure no dose changes are required.  

Untreated hypothyroidism can shorten your dog’s life span, as nearly every organ in the body is affected by thyroid hormone and by the metabolism. Dogs with untreated hypothyroidism develop high cholesterol, decreased immune function, a slowed heart rate, and neuromuscular signs.  

These neuromuscular signs may include unsteadiness, a head tilt, and even seizures. While hypothyroidism responds well to treatment, untreated hypothyroidism can have a negative effect on your dog’s quality of life.  

Recovery and Management of Hypothyroidism in Dogs

Management of hypothyroidism in dogs requires lifelong therapy with oral thyroid hormone replacement. Tolerance of medication may change over time, so your dog may require dose adjustments from time to time. It is recommended that you have your dog's blood thyroid levels checked every 6-12 months to ensure they are still on the appropriate dose of their medication. It is very important that your dog not be given too little or too much thyroid hormone in the long term.  

Once your dog’s thyroid levels have been restored to normal, your dog may lose weight as their body condition improves and will likely have more energy. While it can take months for your dog’s hair to grow back, they will likely experience an improvement in their skin and hair coat over time.  

Hypothyroidism may result in decreased tear production in dogs. Monitor your dog’s eyes for any development of green-yellow discharge. Talk to your veterinarian if you notice any changes in your dog’s eyes.