Hyperthyroidism accelerates several physiologic processes, a fact which is reflected in the decreased systemic vascular resistance, increased cardiac output, increased renal blood flow, hypertrophic and hyperplastic tubuli, and increased glomerular filtration rate.
Causes of canine hyperthyroidism include:
- thyroid adenomas, thyroid carcinoma, ectopic follicular/medullary cell thyroid carcinoma in the mediastinum or heart
- immune-mediated thyroiditis
- dietary hyperthyroidism - induced by feeding a raw meat diet or fed fresh or dried gullets (beef jerky)
- iatrogenic hyperthyroidism - overdosing of levothyroxine for hypothyroidism or Von Willebrand's disease
Approximately 90% of clinically apparent thyroid tumors in the dog are carcinomas. Breeds that have been reported to be at increased risk of thyroid tumors include the Beagle, Golden Retriever and Boxer. benign thyroid tumors in dogs are relatively rare and usually discovered at postmortem consisting of nonfunctional small nodules.
Many hyperthyroid dogs are asymptomatic, but clinical signs can include polyuria (due to loss of medullary hypertonicity), compensatory polydipsia, polyphagia, weight loss, hyperthermia, aggressiveness, tachycardia, panting and restlessness.
Thyroid carcinomas may be detected as a firm mass in the ventral region of the neck; they may be minimally invasive and freely moveable, but more commonly they invade into adjacent structures and are therefore fixed in place.
Chest auscultations may reveal thyrotoxic heart changes such as sinus tachycardia and atrial fibrillation. Additionally, hypertension may be evident using doppler oscillometroscopy (often >180 mmHg).
Diagnosis is based on presenting clinical signs, imaging (ultrasonography or scintigraphy) and demonstration of elevated total thyroxine or free thyroxine (normal 19·3 - 51·5 nmol/L) and thyroid-stimulating hormone (TSH) concentrations (normal < 0·30 ng/mL) using fluoroimmunoassays. However, dogs with thyroid carcinomas are usually euthyroid, but may be hypothyroid or hyperthyroid.
A definitive diagnosis requires thyroid biopsy.
A differential diagnosis in older dogs would include chronic renal disease, diabetes mellitus and hyperadrenocorticism. A differential diagnosis for thyroid carcinoma would include mast cell tumor, lymphoma or metastasis from a malignant oral tumor such as tonsillar squamous cell carcinoma.
Thyroid carcinomas require surgical debulking followed by palliative radiation and/or chemotherapy.
In older canine patients, regularly blood monitoring is recommended to detect early signs of hyperthyroidism.
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