This disease isn’t common in dogs but can lead to some pretty serious health problems.

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The thyroid glands are small glands that do big work in a dog's body. Located on the sides of the neck, these glands regulate a dog's metabolism, the complex process by which the body uses energy from the diet.

The thyroid glands produce several hormones, including thyroxine. With hyperthyroidism, the glands overproduce these hormones, causing a cascade of metabolism-related problems throughout the body.

Although much less common than hypothyroidism, hyperthyroidism in dogs can cause serious health issues if not diagnosed early and treated effectively.

What Causes Hyperthyroidism in Dogs?

The most common cause of hyperthyroidism in dogs is thyroid carcinoma, which is a type of cancer. This cancer causes the thyroid glands to overproduce the hormone thyroxine.

Another cause is overly aggressive treatment of hypothyroidism. Dogs with hypothyroidism take oral medication (usually levothyroxine) to increase thyroxine production. If the dose is too high, the thyroid glands overproduce thyroxine.

Dog Breeds Most Susceptible to Hyperthyroidism

Breeds susceptible to developing thyroid cancer, such as thyroid carcinoma, have a higher risk of hyperthyroidism. These breeds are the Siberian husky, golden retriever, and beagle.

Symptoms of Hyperthyroidism in Dogs

The signs of hyperthyroidism in dogs are related to metabolism that's in overdrive.

Symptoms may be absent in early-stage hyperthyroidism, except for a lump on the side of the neck. They will start to appear as the disease progresses.

The most common signs are weight loss and constant hunger. Other signs of hyperthyroidism include:

  • Hyper-excitability
  • Hyperactivity
  • Vomiting
  • Diarrhea
  • Difficulty breathing
  • Rapid breathing
  • Ruffled fur
  • Poor body condition
  • Excessive drinking and urinating
  • Rapid heart rate
  • Heart murmurs
  • Facial swelling
  • Change in bark
  • Gagging/trouble swallowing
  • Increased defecation

Be aware that these symptoms are not specific to hyperthyroidism. Other diseases can have these same symptoms, so it's important to consult your veterinarian if you notice changes in your pet's health.

dog lying in bed looking up
Credit: Corey O'Hara / Getty

Diagnosing Canine Hyperthyroidism

If your dog has any of the symptoms listed above, take them to your veterinarian.

Your vet will first perform a complete physical exam. Large, palpable masses on the sides of the neck are a big clue for this thyroid disease. After this exam, your vet will take a blood sample for routine blood work and measurement of thyroid hormone levels.

Routine blood work may indicate dehydration and liver changes.

When measuring thyroid hormone levels in the blood, your vet will look specifically at thyroxine levels. Elevated levels indicate hyperthyroidism. But levels may be normal in mild and early-stage hyperthyroidism.

If your dog has normal thyroxine levels but shows symptoms that suggest hyperthyroidism, your vet will want to perform some additional testing to confirm the diagnosis. 

Medication and Treatment for Hyperthyroidism in Dogs

Treatment options for hyperthyroidism in dogs include surgery, radiation therapy, and medication. They can be used alone or in combination. If your dog has hyperthyroidism, your vet will develop a treatment plan to bring your dog's thyroxine levels back to normal.

Surgery

Surgery is recommended for thyroid carcinomas, but only if just one of the thyroid glands is affected. Removal of both glands could lead to hypothyroidism.

Thyroid surgery is complex and typically performed by a specialized veterinary surgeon. Even with a skilled surgeon, the surgery has several complications. For example, the remaining thyroid gland can become hyperactive after the cancerous one has been removed. Also, surgery can damage the tiny parathyroid glands, which control calcium levels. 

Sometimes, thyroxine levels dip too low after surgery, requiring oral thyroid hormone replacement therapy. 

Despite these complications, untreated thyroid carcinoma can spread to other parts of the body, making treatment more difficult.

After surgery, your vet will monitor your dog's thyroxine and calcium levels and prescribe medication as needed to ensure that these levels are within normal ranges.

Radioactive Iodine Therapy

Radioactive iodine therapy is performed more often in cats with hyperthyroidism than dogs with the disease.

Medication

If overly aggressive hypothyroidism treatment has caused hyperthyroidism, the thyroid hormone supplement dose will be lowered. Your vet will monitor your dog's thyroxine levels for the next four to six months to ensure that the supplement's dose is just right.

Hyperthyroidism, although uncommon in dogs, can have negative health consequences if not diagnosed and treated promptly. If you see the signs of this disease in your dog, schedule an appointment with your vet so your pup can get back on the road to feeling better.