Hypothyroidism in Dogs: Symptoms and Treatment

Hypothyroidism in dogs is a common endocrine disease caused by decreased production of thyroid hormones. Because thyroid hormones play an important role in metabolism, low levels can slow down many of a dog’s body functions.

Hypothyroidism is treatable but not curable and is typically managed with lifelong thyroid hormone replacement.

What Is Hypothyroidism in Dogs?

Hypothyroidism in dogs is a common endocrine disease that results in decreased production of thyroid hormones. These hormones are produced by the thyroid glands, which are located on either side of the neck near the throat.

Thyroid hormones play an important role in metabolism. When the glands don't produce 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. Thyroid tumors are fairly uncommon in dogs.

Which Dogs Are More Likely To Develop Hypothyroidism?

Hypothyroidism is most common in middle-aged dogs, with medium- to large-breed dogs more commonly affected.

Breeds that are more predisposed include:

• Golden Retrievers
• Dobermans
• Dachshunds
• Miniature Schnauzers
• Cocker Spaniels
• Irish Setters

What Are the Symptoms of Hypothyroidism in Dogs?

Common symptoms of hypothyroidism in dogs include weight gain without increased appetite, lethargy, heat-seeking behavior, chronic skin and ear infections, and changes to the hair coat.

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

• Weight gain: This often occurs without an increase in appetite. Your dog may gain weight even though they don't eat that much food.
• Lethargy and laziness: Your dog may prefer to sleep and lie around rather than run and play.
• Heat-seeking behavior: Because of low thyroid hormone and the resulting low metabolism, dogs with hypothyroidism may prefer to lie near the fireplace or on a heat vent to 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: A dog with hypothyroidism may lose hair from either side of their back. They may also lose hair from their tail, giving it a “rat tail” appearance.
• Increased pigmentation of the skin
• Inability to regrow hair after it's been shaved

Less Common Signs of Hypothyroidism in Dogs

Some dogs develop less common signs of hypothyroidism, including reproductive problems or nervous system issues such as nerve pain or dragging the hind legs.

Dogs with low thyroid may also have small, white fat deposits on the surface of their eyes or develop dry eye, where they don't produce enough tears.

Some dogs may also develop thickening of the facial skin that causes the muscles of the face to droop.

What Causes 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 it is not clear why these two conditions spontaneously occur in some dogs, there is a genetic predisposition. Another rare cause of hypothyroidism in dogs is cancer, although these other causes are responsible for only a small percentage of hypothyroid patients.

The majority of dogs with hypothyroidism have either inflammation or degeneration of the thyroid gland, and both conditions can be managed with medication.

How Is Hypothyroidism in Dogs Diagnosed?

Hypothyroidism in dogs is diagnosed with blood testing, most commonly starting with a total thyroxine level (Total T4 or TT4). Additional thyroid blood tests, such as a free T4 level or full thyroid panel, may be used to confirm the diagnosis.

Physical Exam and Medical History

Your veterinarian will start with a physical examination and a thorough medical history. It's important to share any unusual behaviors you've noticed in your dog and include a time frame for when you first noticed these behaviors or physical changes.

Blood Work and Urinalysis

Your veterinarian may run basic blood work and a urinalysis to establish your dog's overall health.

Skin Testing

If your dog has skin changes, your veterinarian may perform skin scrapes or smears to rule out secondary skin infections.

A skin scrape involves gently scraping the surface of the skin with a scalpel, while a smear may be collected by pressing a microscope slide to the skin. The samples are then examined under a microscope.

Thyroid Blood Tests

The most common screening test for hypothyroidism is a total thyroxine level (Total T4 or TT4), which determines your dog's main thyroid hormone level.

If the Total T4 is low and your dog has clinical signs of hypothyroidism, this is suggestive of a diagnosis.

Many veterinarians will then run additional blood tests, such as a free T4 level or a full thyroid panel, to confirm the diagnosis.

Some dogs can have a low Total T4 without having hypothyroidism. Occasionally, a dog can have a Total T4 at the low end of the normal range and still have hypothyroidism. Confirmatory testing can be especially helpful in these cases.

How Is Hypothyroidism in Dogs Treated?

Hypothyroidism in dogs is treated with an oral medication called levothyroxine, a synthetic form of the thyroid hormone your dog is missing. Hypothyroidism is treatable but not curable, so your dog will need to stay on thyroid replacement hormone for life.

Levothyroxine comes in several different strengths, and your veterinarian will select the appropriate dose for your dog based on weight. They will likely want to recheck blood work in one month to determine whether any dose changes are needed.

What Happens if Hypothyroidism Is Untreated?

Untreated hypothyroidism can shorten your dog's lifespan, as nearly every organ in the body is affected by thyroid hormone and metabolism.

Dogs with untreated hypothyroidism can develop:

• High cholesterol
• Decreased immune function
• A slowed heart rate
• Neuromuscular signs

Neuromuscular signs may include unsteadiness, a head tilt, and 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 and regular monitoring to ensure your dog is receiving the appropriate dose.

Long-Term Monitoring

Tolerance of medication may change over time, so your dog may require dose adjustments. It is recommended that your dog's blood thyroid levels be checked every six to 12 months to ensure they are still receiving the appropriate dose of medication.

It is very important that your dog not receive too little or too much thyroid hormone in the long term.

What To Expect With Treatment

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.

Monitor Your Dog's Eyes

Hypothyroidism may result in decreased tear production in dogs. Monitor your dog's eyes for the development of green-yellow discharge.

Talk to your veterinarian if you notice any changes in your dog's eyes.