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Hyperthyroidism in Dogs

Hyperthyroidism is a disease caused by overproduction of thyroxine, a thyroid hormone that increases metabolism in the body. The thyroid gland normally produces thyroid hormones in response to stimulation by the pituitary gland, the "master gland" of the body. Thyroid hormones normally increase chemical processes occurring within the cells of the body, especially those related to metabolism; however, in hyperthyroidism, the excessive hormone levels push the cells and body into overdrive, resulting in increased metabolism with concurrent weight loss, anxiety, and diarrhea, among other symptoms.

Hyperthyroidism is rare in dogs, and most commonly occurs as a result of carcinoma of the thyroid. It may also result from medication given to treat hypothyroidism, an underproduction of essential thyroid hormones in the body.

Symptoms and Types

• Involves many organ systems due to the overall increase in metabolism
• Weight loss
• Increased appetite
• Unkempt appearance
• Poor body condition
• Vomiting
• Diarrhea
• Increased thirst (polydipsia)
• Increased urine (polyuria)
• Rapid breathing (tachypnea)
• Difficulty breathing (dyspnea)
• Heart murmur; rapid heart rate; particularly an abnormal heart beat known as a "gallop rhythm"
• Hyperactivity
• Enlarged thyroid gland, which can be felt as a lump on the neck

Some dogs suffering from hyperthyroidism are referred to as apathetic. These patients exhibit atypical signs such as poor appetite, loss of appetite, depression, and weakness.

Causes

• Overfunctioning thyroid nodules (where the thyroid nodules produce excess thyroid hormones outside of the control of the pituitary gland)
• Secretion of the T3 (triodothyronine) or T4 (tetraiodothyronine) thyroid hormones as a result of thyroid cancer; tumor then interferes with normal functioning of the thyroid gland, prompting the gland to produce more thyroxine
• Response to medications used for hypothyroidism may lead to overproduction of thyroxine

Diagnosis

A preliminary diagnosis can often be made based on palpitation of the gland, which becomes enlarged as it progresses. Standard tests will include chemical blood profile, a complete blood count, and a urinalysis. A high concentration of T4 in the blood serum is the most common finding of the profile, confirming a diagnosis of hyperthyroidism. In some cases, however, the T4 levels may be in the normal range, making a diagnosis of hyperthyroidism more difficult. This is especially true in the early stages of this disease. If your dog is showing the symptoms of hypothyroidism but the blood tests are not conclusive, you will need to return to your veterinarian for further blood tests.

Your veterinarian may need to conduct a battery of tests to zero in on a reliable diagnosis. Thyroid gland scintigraphy (a diagnostic test in which a two-dimensional picture of a body radiation source is obtained through the use of radioisotopes) can be used to diagnose hyperthyroidism and to determine the location of abnormal thyroid tissue.

Thoracic radiography and echocardiography may be useful in assessing the severity of myocardial disease, and chest X-rays can be used to detect pulmonary metastasis.