+ Anatomy

• Thyroid glands consist of 2 distinct lobes located lateral and ventral to the 5th-8th tracheal rings
• Thyroid gland in dogs: 50 mm long and 15 mm wide, but larger in immature and brachycephalic dogs
• Isthmus may connect right and left thyroid glands
• Parathyroid glands are salmon-coloured and distinct from the thyroid glands
• 1 external and 1 internal parathyroid gland per thyroid gland
• external parathyroid gland is on the cranial pole of the thyroid gland and external to its capsule
• Internal parathyroid gland are within the thyroid capsule in the caudal and medial aspects of the thyroid
• Cranial thyroid artery is a branch of the common carotid artery and provides the majority of arterial supply to dogs
• Caudal thyroid artery is a branch of the brachiocephalic artery and unites with the cranial thyroid artery in the loose areolar tissue along the dorsomedial surface of the thyroid capsule and provides some arterial supply to dogs
• External parathyroid gland is vascularized by a branch of the cranial thyroid artery
• Venous drainage is through the cranial and caudal thyroid veins
• Innervation: thyroid nerve from the recurrent laryngeal nerve dorsomedial to thyroid glands
• Ectopic thyroid tissue is common and can be located anywhere from the base of the tongue to the heart base

+ General Considerations

• Thyroid gland tumors account for 1.2%-4.0% of all canine tumors
• Median age 9-10 years
• Breed predisposition: Boxer, Beagle, and Golden Retriever
• Etiology unknown although thyroid carcinoma can be induced with exposure to sufficient radiation
• Thyroid adenoma and hyperplasia are usually incidental findings during necropsy
• Malignant thyroid tumors account for 63%-88% of all canine thyroid tumors
• Malignant thyroid tumors are classified as follicular and parafollicular (= medullary or C-cell)
• Follicular thyroid carcinoma is subclassified as compact (solid), follicular, or mixed
• Follicular carcinoma may be very large and invade surrounding normal tissue such as larynx and vasculature
• Majority of dogs with thyroid carcinoma are euthyroid with 10% hyper-functional (± clinical signs)
• Hypothyroidism may also be diagnosed due to destruction of normal thyroid tissue
• Metastatic rate: 33% at diagnosis and 65%-90% during course of disease
• Metastatic sites include regional lymph nodes (i.e., submandibular and retropharyngeal) and lungs, but other sites include adrenal glands, brain, kidneys, heart, liver, and bone
• Thyroid carcinoma can also arise from ectopic tissue in tongue, ventral neck, and cranial mediastinum

+ Clinical Features

• Ventral cervical mass
• 2 broad categories of thyroid carcinoma:
• Freely mobile mass without invasion into surrounding tissue: 33%
• Fixed mass: 67% with dyspnea, dysphonia, dysphagia, and Horner's syndrome

+ General Considerations

• FNA may provide a diagnosis of carcinoma, but blood contamination common
• Definitive diagnosis: biopsy, but this is not recommended preoperatively because of the risk of hemorrhage
• Staging: hematology, serum biochemistry, T 4 concentrations, and thoracic radiographs
• Ultrasound, CT, or MRI are useful in determining extent of soft tissue invasion for large, fixed tumors if surgical resection is planned, or planning of radiation field for radiation therapy
• Nuclear scintigraphy useful in determining function and identifying ectopic malignant tissue
• DDx: abscess or granuloma, salivary mucocele, lymphatic metastasis from oral SCC, LSA, carotid body tumor, and sarcomas such as HSA
• Clinical Staging
• T stage sub-staged as: a = non-fixed, and b = fixed

+ General Considerations

Treatment depends on size of mass, extent of invasion, clinical signs of hyperthyroidism, and availability of different therapies such as radiation therapy and nuclear medicine

+ Surgical Management

• Surgical excision is recommended for freely movable tumors without evidence of deep tissue invasion
• Jugular vein, carotid artery, and vagosympathetic trunk can be sacrificed unilaterally with minimal morbidity
• Surgery can be complicated by local invasion and excessive hemorrhage or regional coagulopathy

+ Radiation Therapy

• Indications: unresectable thyroid masses which are either large, fixed, or invasive
• Fractionated radiation therapy (48 Gy with alternate day therapy):
• Progression-free interval 80% at 1 year and 72% at 3 years
• Metastatic rate 28%
• MST 24.5 months
• Hypofractionated radiation therapy (9 Gy weekly for 4 weeks):
• CR 11% and PR 89%
• MST 96 weeks
• No difference in survival between dogs with and without metastatic disease
• Radiation therapy can be used to down-stage invasive thyroid tumors prior to surgical resection

+ Chemotherapy

• PR 30%-50% in dogs treated with either doxorubicin or cisplatin
• Cisplatin: 9% (1/11) CR and 54% (6/11) PR with overall MST 98 days, but MST for responders 322 days

+ General Considerations

• Prognostic factors:
• Invasion of adjacent tissue is a poor prognostic factor for surgical resection
• Tumor volume > 20 cm 3 or diameter > 5 cm is a poor prognostic factor for metastatic disease
• Bilateral thyroid carcinoma has 16-times greater risk of developing metastatic disease
• Non-medullary thyroid carcinomas may be more likely to develop metastatic disease
• Dogs with no evidence of tumor progression have 15-times decreased risk of developing metastasis