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Hyperthyroidism in Dogs

What Is Hyperthyroidism in Dogs?

Hyperthyroidism in dogs is a seldom-seen ailment resulting from an excess production of hormones by the thyroid gland situated in the neck region. Typically, this condition stems from a malignant tumor growing within the thyroid. However, benign thyroid masses, dietary factors, and specific supplements can also play a role.

What Does the Thyroid Do?

The thyroid gland, resembling a butterfly, resides on either side of a dog’s windpipe, known as the trachea. Its primary function is to produce thyroid hormones, essential for metabolism and calcium regulation. In cases of hyperthyroidism, the thyroid gland becomes excessively stimulated, resulting in the production and release of large quantities of thyroid hormones into the body. Consequently, this surplus of thyroid hormones accelerates the dog’s metabolism and elevates calcium levels.


Dogs with hyperthyroidism exhibit symptoms stemming from their heightened metabolism, including hyperactivity and weight loss despite an insatiable appetite. Elevated calcium levels associated with this condition can lead to the following manifestations:

  • Increased thirst
  • Increased urination
  • Decreased energy levels
  • Vomiting
  • Difficulty defecating (constipation)

If the underlying cause of hyperthyroidism is a cancerous mass within the thyroid gland, it may be detectable by touch in the neck region. This tumor can also result in symptoms such as coughing, difficulty breathing, challenges in eating, and alterations in the dog’s bark tone.


The primary cause of hyperthyroidism in dogs is typically attributed to the presence of a cancerous mass in the thyroid gland, which can manifest as either a thyroid carcinoma or thyroid adenocarcinoma, leading to excessive production of thyroid hormones. These thyroid masses are usually localized within the thyroid gland, although instances of thyroid tissue occurring in abnormal locations, termed ectopic thyroid tissue, have been observed. This ectopic tissue can be found under a dog’s tongue or at the base of the heart, potentially resulting in thyroid tumors and subsequent hyperthyroidism. In rare cases, hyperthyroidism can also arise from the presence of a benign mass known as a thyroid adenoma within the thyroid gland.

Additionally, hyperthyroidism may develop in dogs consuming commercially available raw meat diets contaminated with thyroid tissue or through the ingestion of supplements containing kelp or seaweed. Overmedication with thyroid supplements, particularly in cases of hypothyroidism (underactive thyroid), can also contribute to the onset of hyperthyroidism.

While hyperthyroidism can affect any breed of dog, certain breeds, including Beagles, Boxers, Golden Retrievers, and Siberian Huskies, exhibit a higher incidence of developing thyroid tumors. Furthermore, hyperthyroidism predominantly occurs in older dogs.


To diagnose hyperthyroidism in dogs, veterinarians typically employ a series of diagnostic procedures, including:

  • Routine physical examination: During this examination, the veterinarian may palpate a mass in the neck region or identify a mass under the tongue. They may also observe signs such as an elevated heart rate, increased temperature, weak pulses, or rapid weight loss. Enlarged lymph nodes may or may not be detected during the examination.
  • Comprehensive bloodwork: This includes a complete blood cell count (CBC), a chemistry panel including calcium levels, and a thyroid panel. Abnormal thyroid levels and elevated calcium levels are common findings in the bloodwork of dogs with hyperthyroidism.
  • Urinalysis: Dogs with hyperthyroidism often exhibit increased water intake and urination frequency. Urinalysis helps rule out common causes such as urinary tract infections, kidney disease, and diabetes. Urinalysis results for dogs with hyperthyroidism typically appear normal.
  • Ultrasound of the neck: If a palpable mass is detected in the neck region, ultrasound is used to ascertain whether the mass is within the thyroid gland and to determine its size.
  • Ultrasound of the heart (Echocardiography): This may be necessary to check for an ectopic thyroid mass located at the base of the heart.
  • CT scan or MRI: Advanced imaging techniques may be required to measure the mass accurately and determine if it has adhered to surrounding tissue. This helps assess if the mass is surgically removable.
  • Thyroid scintigraphy: Also known as a thyroid scan, this diagnostic test provides visualization of the thyroid gland to detect the presence of a mass.

Biopsy or fine needle aspiration and cytology of a thyroid mass are generally not recommended due to the high vascularity of the thyroid gland, which can result in inconclusive samples due to the presence of red blood cells. Additionally, there’s a risk of severe hemorrhage if a needle or biopsy tool is inserted into a thyroid tumor.


  • Surgery: Surgical removal of the thyroid tumor, known as thyroidectomy, is recommended if the tumor is not attached to surrounding tissues. This procedure can also address tumors located under the tongue or at the base of the heart. In cases where immediate surgery isn’t feasible, the veterinarian may prescribe methimazole, an oral medication, to manage thyroid levels until surgery can be performed.
  • Debulking and adjuvant therapy: If the thyroid tumor is attached to surrounding tissues, complete surgical removal may not be possible. In such instances, a surgeon may remove a significant portion of the tumor (debulking) followed by chemotherapy and/or radiation to target any remaining tumor cells. Aspiration pneumonia is a common complication post-thyroidectomy, necessitating close monitoring for signs of respiratory distress in the patient pet.
  • Radiation therapy: Radiation may be pursued if the cancer has metastasized or if surgery poses risks due to the size or invasiveness of the tumor.
  • High-dose radioactive iodine (I-131) therapy: This treatment is effective for cancerous thyroid masses that cannot be surgically removed or have spread. It’s also used to target any residual cancer cells post-thyroidectomy.
  • Chemotherapy: Drugs such as toceranib phosphate (Palladia), doxorubicin, and cisplatin have shown efficacy in treating hyperthyroidism caused by cancerous thyroid masses.
  • Dietary changes: For dogs with hyperthyroidism triggered by consuming commercial raw meat diets with thyroid tissue or supplements containing seaweed or kelp, discontinuing the diet or supplement is recommended.
  • Adjustment of thyroid supplement dosage: Dogs receiving excessive thyroid supplementation for hypothyroidism management require dosage reduction to prevent hyperthyroidism symptoms.
  • Palliative care: In cases where surgery, chemotherapy, or radiation isn’t pursued, palliative treatment involves using methimazole medication and Hill’s y/d diet to manage hyperthyroidism symptoms by reducing thyroid hormone production. While this approach provides symptom relief, it doesn’t address the thyroid tumor’s growth or potential spread to other areas of the body.

Living and Management

The recovery and management of hyperthyroidism in dogs depend on the underlying cause and treatment approach:

  • Prognosis:
      • If hyperthyroidism is caused by a benign tumor, surgical removal usually leads to a complete cure.
      • Hyperthyroidism triggered by dietary factors or supplements can be resolved by discontinuing these sources.
      • For hypothyroid dogs receiving excessive thyroid supplements, reducing the dosage typically resolves hyperthyroid symptoms.
  • Treatment of Cancerous Thyroid Tumors:
      • Surgical removal of movable tumors not adhered to surrounding tissue can extend the dog’s life by approximately three years.
      • Large tumors or those adhered to surrounding tissue may not be fully removable surgically, requiring additional treatments to manage cancer.
      • Radiation or radioactive iodine therapy for non-metastatic tumors can offer a survival time of one to three years.
  • Metastasis:
      • Cancerous thyroid tumors often spread to other areas, such as regional lymph nodes, reducing prognosis.
      • Metastasis typically necessitates chemotherapy to target cancer in multiple body locations, with a survival time of less than a year with treatment.
  • Post-Thyroidectomy Management:
    • Dogs undergoing thyroidectomy may become hypothyroid post-surgery, requiring long-term thyroid supplementation.
    • Some dogs may experience low calcium levels after surgery and may need calcium supplements to manage this condition.

Hyperthyroidism in Dogs FAQs

Which dog breeds are more prone to hyperthyroidism?

While hyperthyroidism can affect any breed of dog, Beagles, Boxers, Golden Retrievers, and Siberian Huskies have a higher incidence of developing thyroid tumors.

What is a dog goiter?

A dog goiter refers to an enlargement of the thyroid gland, often due to benign conditions like congenital hypothyroidism or as a side effect of trimethoprim-sulfa, an oral antibiotic. Once the underlying cause is addressed, the goiter typically resolves.

What is the life expectancy of dogs with hyperthyroidism?

Dogs with non-adhered, cancerous thyroid tumors can have a life expectancy of up to three years if they undergo treatments such as surgery, radiation therapy, or radioactive iodine therapy. However, if metastasis is present, survival time with treatment options like surgery, radiation therapy, chemotherapy, and/or radioactive iodine therapy is typically less than a year.

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