Understanding Hyperthyroidism in Dogs: Causes, Symptoms, and Treatments

Hyperthyroidism, a condition characterized by the overproduction of thyroid hormones, is a rare but serious ailment in dogs. While more common in cats, it can significantly impact a dog’s health, primarily affecting their metabolism and calcium levels. Understanding the intricacies of this disease is crucial for timely diagnosis and effective management. This condition is most often caused by a cancerous tumor within the thyroid.

The thyroid gland, a small, butterfly-shaped organ situated on either side of a dog’s windpipe, plays a vital role in regulating metabolism and calcium. When this gland becomes overstimulated, it releases excessive amounts of thyroid hormones, leading to an accelerated metabolism and elevated calcium levels. This hormonal imbalance can manifest in a variety of symptoms, necessitating a thorough diagnostic approach by veterinarians.

Symptoms of Hyperthyroidism in Dogs

The most prominent symptom of hyperthyroidism in dogs is a significantly increased metabolism, often leading to hyperactivity and unexplained weight loss, even when the dog exhibits a voracious appetite. In addition to these, elevated calcium levels can contribute to several other clinical signs:

• Increased thirst and urination: Dogs may drink and urinate much more frequently than normal.
• Decreased energy levels: Despite the hyperactivity associated with an increased metabolism, some dogs may paradoxically show a decline in overall energy.
• Vomiting: Gastrointestinal upset, including vomiting, can occur.
• Constipation: Difficulty in defecating is another potential symptom.

In cases where hyperthyroidism is caused by a cancerous thyroid mass, the tumor itself may be palpable in the neck region. This can lead to additional symptoms such as coughing, difficulty breathing, problems with eating, and a noticeable change in the dog’s bark.

Causes of Hyperthyroidism in Dogs

The predominant cause of hyperthyroidism in dogs is a cancerous thyroid mass, typically a thyroid carcinoma or thyroid adenocarcinoma, which autonomously produces excess thyroid hormones. These tumors usually originate within the thyroid gland itself. However, aberrant thyroid tissue, known as ectopic thyroid tissue, can sometimes be found in unusual locations, such as under the dog’s tongue or at the base of the heart. Consequently, thyroid tumors can occasionally develop in these ectopic sites, leading to hyperthyroidism. Less commonly, a benign mass, a thyroid adenoma, within the thyroid gland can also trigger the condition.

Dietary factors can also play a role. Dogs that consume commercial raw meat diets contaminated with thyroid tissue are at risk of developing hyperthyroidism. Similarly, supplements containing kelp or seaweed can precipitate the condition if ingested. Furthermore, hyperthyroidism can arise from an overdose of thyroid supplements in dogs being treated for hypothyroidism (an underactive thyroid).

While any dog breed can be affected, certain breeds have a higher predisposition to developing thyroid tumors, including Beagles, Boxers, Golden Retrievers, and Siberian Huskies. The condition most frequently affects older dogs.

Veterinary Diagnosis of Hyperthyroidism in Dogs

Diagnosing hyperthyroidism in dogs involves a comprehensive approach, typically including:

• Routine Physical Examination: A veterinarian may detect a palpable mass in the neck or under the tongue. Other findings can include an elevated heart rate, increased body temperature, weak pulses, or rapid weight loss. Enlarged lymph nodes may or may not be present.
• Comprehensive Bloodwork: This includes a complete blood count (CBC), a chemistry panel to assess calcium levels, and a thyroid panel. Bloodwork often reveals abnormal thyroid hormone levels and elevated calcium.
• Urinalysis: To rule out other common causes of increased thirst and urination such as urinary tract infections, kidney disease, and diabetes, a urinalysis is performed. In hyperthyroid dogs, urinalysis results are typically normal.
• Ultrasound of the Neck: If a neck mass is detected, ultrasound helps confirm if it is within the thyroid gland and determine its size.
• Ultrasound of the Heart (Echocardiogram): This may be necessary to investigate for ectopic thyroid masses near the heart.
• CT Scan or MRI: Advanced imaging may be employed to precisely measure the mass and assess its invasiveness into surrounding tissues, aiding in surgical planning.
• Thyroid Scintigraphy: This diagnostic imaging technique can visualize the thyroid gland to identify the presence and extent of any masses.

Fine needle aspirates or biopsies of thyroid masses are generally not recommended due to the gland’s high vascularity, which can lead to inconclusive results from blood contamination and a risk of severe hemorrhage.

Treatment Options for Hyperthyroidism in Dogs

Treatment strategies for canine hyperthyroidism depend on the underlying cause and the specific condition of the tumor:

• Surgery (Thyroidectomy): If a thyroid tumor is identified and has not invaded surrounding tissues, surgical removal of the tumor is the recommended treatment. Surgery can also address masses found under the tongue or at the base of the heart. In cases where immediate surgery isn’t feasible, oral medication like methimazole may be prescribed to manage thyroid levels until the procedure. If the tumor is extensively adhered to surrounding tissues, surgical debulking followed by chemotherapy or radiation may be necessary. A common post-operative complication is aspiration pneumonia, requiring close monitoring of respiratory function.
• Radiation Therapy: This may be considered for metastatic cancer or when surgery poses too high a risk due to the tumor’s size or invasiveness.
• High-Dose Radioactive Iodine (I-131) Therapy: An effective treatment for cancerous thyroid masses that are inoperable or have metastasized, as well as for residual cancerous cells after surgery.
• Chemotherapy: Several chemotherapy drugs, including toceranib phosphate (Palladia), doxorubicin, and cisplatin, have shown efficacy in treating hyperthyroidism caused by cancerous thyroid masses.
• Dietary Adjustments: For hyperthyroidism stemming from diets containing thyroid tissue or supplements with kelp/seaweed, discontinuing these dietary items is the primary treatment.
• Thyroid Supplement Dosage Adjustment: Overmedication with thyroid supplements for hypothyroidism requires a dosage reduction to resolve hyperthyroid symptoms.
• Palliative Care: For owners opting against aggressive treatments, medications like Methimazole combined with a specific diet (Hill’s y/d) can help manage symptoms by reducing thyroid hormone production. This approach provides symptomatic relief but does not address the tumor’s growth or potential spread.

Recovery and Management of Hyperthyroidism in Dogs

The prognosis for hyperthyroidism in dogs varies significantly based on the cause. Benign tumors, if surgically removed, are often curative. Conditions related to diet or supplements are resolved by eliminating the offending agent, and dosage adjustments for overmedicated hypothyroid dogs will resolve hyperthyroid symptoms.

Managing cancerous thyroid tumors presents greater challenges. If a tumor is movable and not deeply invasive, surgical removal can extend a dog’s life by approximately three years. However, large or invasive tumors may not be completely removable, necessitating further treatment. Radiation or radioactive iodine therapy for non-metastatic tumors can also lead to a survival time of one to three years.

Thyroid tumors have a high propensity for metastasis, spreading to nearby lymph nodes. If metastasis is present, the prognosis is generally poor, with survival typically less than a year even with aggressive treatments like surgery, radiation, and chemotherapy.

Following thyroidectomy, some dogs may develop hypothyroidism and require lifelong thyroid hormone supplementation. Additionally, low calcium levels post-surgery may necessitate calcium supplementation.

Frequently Asked Questions about Hyperthyroidism in Dogs

What dog breeds are predisposed to hyperthyroidism?
While any breed can develop hyperthyroidism, Beagles, Boxers, Golden Retrievers, and Siberian Huskies show a higher incidence of thyroid tumors.

What is a dog goiter?
A goiter is an enlarged thyroid gland resulting from benign conditions like congenital hypothyroidism or as a side effect of certain antibiotics. It typically resolves once the underlying cause is treated.

What is the life expectancy of dogs with hyperthyroidism?
For dogs with a non-adhered cancerous thyroid tumor, life expectancy can extend up to three years with treatments like surgery, radiation, or radioactive iodine therapy. If metastasis has occurred, survival time is usually less than a year, even with comprehensive treatment.

References

1. Exogenous thyrotoxicosis in dogs attributable to consumption of all-meat commercial dog food or treats containing excessive thyroid hormone: 14 cases (2008-2013). J Am Vet Med Assoc. January 2015;246(1):105-11.
2. Hyperthyroidism in dogs caused by consumption of thyroid-containing head meat. Schweiz Arch Tierheilkd. February 2013;155(2):149-52.
3. Nadeau M-E, Kitchell BE. Evaluation of the use of chemotherapy and other prognostic variables for surgically excised canine thyroid carcinoma with and without metastasis. Canadian Veterinary Journal. 2011;52:994–998.
4. Dietary Management of Hyperthyroidism in a Dog. J Am Anim Hosp Assoc. 2017;53(2):111-118.
5. Veterinary Information Network. Thyroid Neoplasia (Canine).