Hyperthyroidism in Dogs, what is it?

  • By: Mick Whitefield
  • Time to read: 7 min.

Hyperthyroidism in dogs is a rare condition where the thyroid glands generate an excess of thyroid hormones leading to a constant state of metabolic hyperactivity. In other words, the thyroid glands are in overdrive.

Located in the neck, the thyroid gland produces the hormone thyroxine (T4) and several other essential thyroid hormones. All these hormones play an important part in the dog’s metabolism and, when not produced at normal levels, can cause major problems. The thyroid is comparable to a thermostat of the dog’s body.

Thyroid Glands In Dog

Is Hyperthyroidism Dangerous in Dogs?

Hyperthyroidism is a rather rare condition commonly seen in older dogs. In Hyperthyroidism, excessive hormone levels push the body into overdrive leading to an increased metabolism characterized by anxiety, weight loss, diarrhea, among other symptoms.

Hyperthyroidism is not the same condition as Hypothyroidism.


Dogs with thyroid complications usually suffer from Hypothyroidism, a condition in which the glands are not producing adequate amounts of the thyroid hormone. In Hypothyroidism, the dog suffers from an underactive thyroid condition. Hypothyroidism causes the dog’s bodily functions to slow down. 

For treating and managing Hyperthyroidism in dogs, various options are available. If the cause is a tumor, options include radiation, chemotherapy, and surgery. Based on the diagnosis, your vet will determine and recommend the best course of medical action.

What are the Causes of Hyperthyroidism?

Hyperthyroidism is usually caused by multiple factors. A major cause of Hyperthyroidism is over-functioning thyroid nodules. The thyroid nodules produce excess amounts of thyroid hormones beyond the control of the dog’s pituitary gland.

Secretion of the Triodothyronine (T3) or Tetraiodothyronine (T4) thyroid hormones is another factor that results in thyroid cancer. The ensuing tumor interferes with thyroid gland normal functioning, prompting the gland to produce even more thyroxine.

In some cases of canine Hyperthyroidism, the thyroid hormone overproduction doesn’t originate in the thyroid itself. The medications administered for Hypothyroidism contain a synthetic form of thyroxine. Sometimes, an overcorrection of hormone levels can lead to Hyperthyroidism in dogs.

Thyroid Glands Hyperthyroidism

Signs and Symptoms of Hyperthyroidism?

Although rare, Hyperthyroidism is a very serious health concern for dogs. If you notice any of the following symptoms, we recommend you immediately contact your vet. In the early stages of the condition, many dogs will show no signs or just have a lump on the underside of the neck.  

Symptoms of hyperthyroidism in dogs include:

  • Weight loss despite not showing any change in appetite
  • Excessive water consumption
  • Heart murmur
  • Increased heart rate
  • Tachycardia (fast heart rate)
  • Dyspnea (shortness of breath)
  • Cardiomyology (enlarged heart)
  • Congestive heart failure (CHF)
  • Increased amount of stool
  • Hyperactivity
  • Diarrhea and vomiting
  • Unkempt appearance
  • Poor body condition

Unfortunately, most of these signs aren’t necessarily specific to thyroid conditions disease, and symptoms may vary from case to case. The best option is to consult the vet as soon as you notice any of the above signs. 

Less commonly recognized signs of Hyperthyroidism seen in a small number of dogs include: 

  • Megaesophagus (dilation of the esophagus) that causes regurgitation.
  • Abnormal function of muscles or nerves leading to weakness to walk.

Although rare in dogs, if left untreated, Hyperthyroidism can cause kidney and heart failure.

What dog breeds are prone to hypothyroidism?

Some dog breeds seem to be at greater risk of developing Hypothyroidism than others. Vulnerable breeds that have an increased risk of thyroid cancer include:

  • Siberian Husky
  • Golden Retriever
  • Beagle
  • Boxers

Cancer-associated Hyperthyroidism is commonly seen in middle-aged to older (>9 years) larger breed dogs. 

How do Vets Diagnose Hyperthyroidism in Dogs?

Your vet will diagnose the condition based on careful diagnostic testing and clinical signs. This may necessitate several blood tests to determine if the animal exhibits any of the clinical abnormalities commonly associated with Hyperthyroidism. The vet will also monitor your pet’s thyroid levels.

Several blood tests can be used to confirm the presence of a suspected diagnosis of the disease. Often, blood testing is performed as a panel of several tests to boost the yield of such tests. Please note that the results of some of these blood tests can be influenced or affected by the presence of several other non-thyroid diseases. Therefore, the test results need to be considered in light of the broader picture.

During the examination, your vet may detect an increased heart rate or feel a thyroid mass in the animals’ neck. To confirm the physical hyperthyroidism diagnosis, it may be necessary to conduct some blood work to measure thyroid hormone levels. Following an initial diagnosis, the vet may recommend further testing to help determine the extent or spread of the disease and the most suitable treatment for your dog. 

The earlier the treatment begins, the better are the chance of recovery for your 4-legged buddy. In some cases, it may be necessary to make dietary restrictions after the dog recovers, especially to preserve the kidney’s health.

Medical Treatment of Hyperthyroidism

If Hyperthyroidism is strictly a thyroid issue and not the result of thyroid carcinoma, several medications can help control the condition and bring the thyroid levels back to normal. Better nutritional management is also helpful.

However, if hyperthyroidism results from thyroid carcinoma, the cure will depend on several factors, including the stage in which the condition is caught. If detected early, the odds of a full recovery are better, but the recovery prognosis is poor if discovered when the tumor mass is much larger.

Once the vet has initiated treatment, he will need to have the dog re-examined every two to three weeks for the initial treatment period lasting three months. A complete blood count will also be carried out to check for thyroid hormone concentration (T4). The medication dosage will be adjusted to help maintain the T4 concentration in the normal range.

Dog With Cannula Small

If the vet suspects a thyroid tumor, testing for cancer spread (metastasis) can include abdominal ultrasound and/or chest x-rays. Advanced imaging techniques like computed tomography (CT) or magnetic resonance imaging (MRI) scans may be done to see the size of the tumor and whether it could be invading important structures such as the carotid artery. 

Treatment may involve partial removal of the tumorous mass or removing the entire thyroid gland. However, that will depend on what the vet deems to the most appropriate line of action. The use of radioactive therapy (radioiodine) as a mode of treatment is restricted and confined to a medical facility, as it can be radioactive in itself. Even after radioactive therapy, you will still need to take precautions after bringing the dog home to reduce the risk of toxic reactions to the radioactive treatment. The vet will advise you on the best preventive measures. 

If the dog has had surgery, your vet will closely observe the physical recovery. In the first week following surgery, the vet will measure thyroid hormone levels and monitor thyroid gland activity every 3-6 months after that.

How to Prevent Thyroid disease in Pets

There is not much you can do to prevent thyroid tumors in pets. However, if hyperthyroidism resulted from a medication overdose, giving medication exactly as prescribed by the vet and frequent rechecks will help avoid additional overdoses.

If diagnosed with Hypothyroidism, remember that administering a high dose of hypothyroid medication to your dog can have the same effect as an overactive thyroid gland. Be aware of the symptoms associated with Hyperthyroidism, and immediately consult the veterinarian if you notice any of them in your dog. The vet will help correct the diagnosis and medication.

Paralysis of the voice box and/or development of low blood-calcium levels during the initial postoperative period are some of the complications that need to be watched for and, should be treated, if they occur. 

Hyperthyroidism what food to avoid

To help prevent Hyperthyroidism, a healthy diet is important for your dog. A raw food diet can be very good for your dog but;

It’s worth noting that certain ingredients like animal necks, head meat, and gullets can trigger Hyperthyroidism as they contain higher amounts of thyroid hormones. (See Dietary Hyperthyroidism in dogs, by B Kohler,C Stengel and R Neiger).

Consult your vet before you switch your pet to a diet of raw food. 

Highly processed foods are not recommended if your dog has thyroid disease. They can contain imbalanced omega 6-3 ratios, a high percentage of carbohydrates, and other inappropriate ingredients, all known to cause body inflammation. Some processed foods put huge pressure on the dog’s digestive system, liver, and pancreas.

Whenever possible, feed your dog with whole fresh foods as they provide a wide variety of nutrients that ensure a healthier life and a better chance of living longer.

Conclusion of Hyperthyroidism in Dogs

Hyperthyroidism in dogs is rare, but have him checked right away if you notice any signs and symptoms. When in doubt, consult your vet because Hyperthyroidism can lead to bigger health problems and a poor quality of life when left untreated. Even if it’s just a small lump around the dog’s throat region, get him to the veterinarian for an immediate evaluation.

If diagnosed and treated early, the prognosis for your dog is generally good. With timely and appropriate treatment, many dogs get better and go on to enjoy a long life. 

For dogs with Hyperthyroidism, fresh food is recommended, preferably lightly cooked. Make sure that the medication doesn’t lead to an overcorrection which could trigger Hyperthyroidism. 

Reference: Dietary Hyperthyroidism in dogs, by B Kohler,C Stengel and R Neiger

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