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.