Veterinary Information:
Hyperthyroidism in Dogs

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Terminology: Malignant thyroid tumors are fairly common in dogs. These tumors are however rarely functional (e.g. produce thyroid hormones)and therefore seldom result in hyperthyroidism. In middle-aged to old cats (median age 13 years) it is the most common endocrine condition. An excessive amount of circulating thyroid hormones results in multi-systemic disorders.

Aetiology:
Excessive amounts of circulating thyroid hormones in dogs and cats are generally the result of functional thyroid neoplasias or hyperplasias.

In dogs thyroid tumors are fairly common (up to 4% of all tumor cases in dogs). Most of these thyroid tumors are invasive and non-functional carcinomas. These carcinomas do not produce thyroid hormones and therefore do not result in hyperthyroid dogs. The invasive and destructive character of these nonfunctional carcinomas can often result in canine hypothyroidism. Thyroid carcinomas can invade locally in surrounding musculature, vessels, nerves, trachea and esophagus. Distant metastases to the longs are also common. 10% of the thyroid carcinomas in dogs are autonomous hyperfunctional, resulting in excessive amounts of circulating thyroid hormones in dogs. Adenomatous (benign) thyroid hyperplasias, the main cause of this condition in cats, are very rare in dogs.

Predisposition:
Age: mostly middle-aged or older dogs (older than 4 years of age).
Both in males and females.
Breeds: Boxers, Golden Retrievers and Beagles.

Symptoms:
-an enlarging mass in the ventral cervical region
-polyuria and polydipsia (pu/pd) (common first signs)
-weight loss despite good appetite (common first signs)
-weakness
-fatique
-nervousness / restless
-heat intolerance
-frequent defecation
-forceful arterial pulse
-forceful apex beat
-high voltage ECG
-possible complications of malignancy: dyspnoe, dysphagia

Differential Diagnoses:
Enlarged ventral mass in the cervical region due to:
-inflammation (granuloma, abcess)
-non-functional thyroid tumor
-non-thyroid neoplasia in the cervical region (lymphoma, metastases).

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