Canine Hyperthyroidism Study Guide

Overview and Clinical Importance

Hyperthyroidism is a relatively rare endocrine disorder in dogs, occurring far less commonly than in cats. Unlike feline hyperthyroidism, which is typically caused by benign adenomatous hyperplasia, canine hyperthyroidism is almost always caused by thyroid carcinoma, a malignant neoplasm of the thyroid gland. This makes canine hyperthyroidism clinically distinct and carries significant prognostic implications that are high-yield for the NAVLE.

The thyroid gland in dogs consists of two separate lobes located lateral and slightly ventral to the proximal trachea, typically spanning the first to eighth tracheal rings. The glands are highly vascular, receiving blood supply from the cranial and caudal thyroid arteries. Understanding this anatomy is critical for surgical planning and interpreting imaging findings.

Etiology and Pathophysiology

Primary Causes

Thyroid Carcinoma (Most Common)

Thyroid carcinoma accounts for approximately 90% of thyroid tumors in dogs. These malignant neoplasms arise from either follicular cells (follicular thyroid carcinoma, FTC) or parafollicular C-cells (medullary thyroid carcinoma, MTC). Importantly, only 10-20% of thyroid carcinomas are functional (produce excess thyroid hormone), meaning most dogs with thyroid tumors are actually euthyroid or even hypothyroid at presentation.

Classification of Canine Thyroid Tumors

Other Causes of Canine Hyperthyroidism

Iatrogenic hyperthyroidism: Overdosing levothyroxine in dogs being treated for hypothyroidism is a common cause. Clinical signs resolve when medication is discontinued or dose is reduced.

Dietary hyperthyroidism: Raw food diets containing excessive thyroid gland tissue (from livestock gullets) can cause thyrotoxicosis. This resolves with diet change.

Exogenous exposure: Rare cases of coprophagia from housemates receiving levothyroxine supplementation have been reported.

Breed Predisposition and Signalment

Clinical Presentation

Clinical Signs of Hyperthyroidism

Clinical hyperthyroidism occurs in only 10-20% of dogs with thyroid tumors. When present, signs reflect the hypermetabolic state caused by excess T4 and T3:

Signs Related to Thyroid Mass Effect

Most dogs with thyroid tumors present due to local mass effects rather than systemic hyperthyroidism:

• Palpable cervical mass: Most common presentation; located in ventral to ventrolateral neck
• Dysphagia: Difficulty swallowing due to esophageal compression
• Dysphonia: Voice change from recurrent laryngeal nerve involvement
• Coughing/gagging: Tracheal compression
• Respiratory distress: Severe tracheal or laryngeal compression
• Regurgitation: Megaesophagus secondary to nerve involvement (rare)

Diagnostic Approach

Initial Evaluation

Physical Examination Findings

• Palpable cervical mass: Assess mobility (critical for treatment planning)
• Mobile tumor: Can be moved greater than 1 cm in all planes; better surgical candidate
• Fixed tumor: Adhered to underlying structures; indicates invasiveness
• Tachycardia: If functional tumor present
• Heart murmur: Secondary to thyrotoxic heart disease

Laboratory Testing

Diagnostic Imaging

Cervical Ultrasound

Cervical ultrasound is the first-line imaging modality for evaluating thyroid masses. It allows assessment of: tumor size and location, echogenicity, presence of cystic areas or necrosis, relationship to major vessels (carotid artery, jugular vein), and regional lymph node involvement.

Computed Tomography (CT)

CT scan provides superior detail for surgical planning and staging. Pre-contrast scans show tumor location; post-contrast scans demonstrate vascular supply and invasion. CT of the neck, thorax, and abdomen allows complete staging in one session.

Nuclear Scintigraphy

Technetium-99m pertechnetate scintigraphy is valuable for: determining functional status, identifying ectopic thyroid tissue (present in 23-80% of dogs), detecting metastatic disease, and determining candidacy for radioactive iodine therapy. Tumors that concentrate radionuclide may respond to I-131 therapy.

Thoracic Radiographs

Essential for staging to identify pulmonary metastases. Approximately 35-40% of dogs have evidence of metastasis at diagnosis, with lungs being the most common site.

Cytology and Histopathology

Fine needle aspirate (FNA): Can provide presumptive diagnosis of neuroendocrine neoplasia. However, caution is advised due to high vascularity of thyroid tumors, which may cause significant hemorrhage. Ultrasound-guided FNA is preferred.

Histopathology: Definitive diagnosis requires histopathologic examination. Immunohistochemistry (thyroglobulin for FTC, calcitonin for MTC) helps differentiate tumor types.

Exam Focus: The most important pre-surgical assessment is determining whether the tumor is MOBILE or FIXED. Mobile tumors have excellent prognosis with surgery (MST greater than 3 years). Fixed tumors require radiation therapy or I-131 as first-line treatment.

WHO Clinical Staging (TNM)

Treatment Options

Surgical Thyroidectomy

Surgery is the treatment of choice for mobile, non-metastatic thyroid carcinomas. Only 25-50% of canine thyroid carcinomas are amenable to complete surgical resection due to local invasiveness and proximity to critical structures.

Surgical Complications

• Hemorrhage: Thyroid tumors are highly vascular; meticulous hemostasis essential
• Hypothyroidism: Post-bilateral thyroidectomy; requires levothyroxine supplementation
• Hypocalcemia: From parathyroid removal or devascularization; monitor ionized calcium post-operatively
• Laryngeal paralysis: Recurrent laryngeal nerve damage
• Horner syndrome: Vagosympathetic trunk damage

Radiation Therapy

External beam radiation therapy (EBRT) is recommended for fixed, invasive tumors not amenable to surgery. The protocol typically involves 20 treatments (Monday-Friday for 4 weeks) under daily anesthesia. Complete response (tumor resolution) is seen in 8% of dogs, partial response (greater than 50% reduction) in 69%. Some tumors may become surgically resectable after radiation-induced shrinkage.

Radioactive Iodine (I-131) Therapy

I-131 therapy is effective for functional tumors that concentrate iodine on scintigraphy. It can be used as sole treatment or adjunct to surgery for residual/metastatic disease. Dogs require higher doses than cats (555-1850 MBq vs. 2-5 mCi) and prolonged isolation due to radiation safety concerns. MST of 30-34 months reported with I-131 alone or combined with surgery.

Chemotherapy

The role of chemotherapy remains poorly defined. Response rates of 30-50% have been reported with doxorubicin or carboplatin, but improved survival times have not been consistently demonstrated. Chemotherapy is typically considered for metastatic disease or high-risk tumors (large, bilateral, vascular invasion). Toceranib phosphate (Palladia), a tyrosine kinase inhibitor, has shown promise in some cases with tumor stabilization or shrinkage.

Medical Management of Hyperthyroidism

Methimazole (or carbimazole) can control clinical signs of hyperthyroidism but does NOT treat the underlying tumor. It blocks thyroid hormone synthesis, providing palliative control. The tumor will continue to grow. Median survival with medical management alone is approximately 15 months. This is primarily used for pre-operative stabilization of hyperthyroid patients or palliative care when definitive treatment is declined.

Treatment Decision Algorithm

Key question: Is the tumor MOBILE or FIXED?

Prognosis

Negative Prognostic Factors

• Fixed/invasive tumor (vs. mobile)
• Bilateral thyroid involvement
• Gross or histologic vascular invasion
• Large tumor size (greater than 5 cm)
• Presence of distant metastasis at diagnosis
• Undifferentiated/anaplastic histologic type
• Incomplete surgical margins