The Causes of Canine Hyperthyroidism

Canine hyperthyroidism is a disease of the dog’s endocrine system. Although rare, it usually occurs in geriatric dogs whose systems are no longer as balanced as they should be. A dog with hyperthyroidism appears excessively hungry, wolfing down his food sometimes to the point of vomiting -- but he continues to lose weight. Other symptoms include increased thirst, frequent urination, rapid breathing, restlessness, hyperactivity, irritability and a dull-looking, untidy coat.

A carcinoma, or cancerous tumor, of the thyroid gland is the main cause of hyperthyroidism in dogs. The dog develops a swelling of the throat that affects his ability to breathe and swallow, and results in coughing and hoarseness when he barks. This causes the overproduction of thyroxine, which results in the condition hypothyroidism. Boxers, beagles, golden retrievers and Labrador retrievers have a higher risk of developing thyroid tumors, particularly past the age of 5 years. The average age of patients, though, is 10 years, according to Dr. Mark Peterson of the Animal Endocrine Clinic.

Thyroid nodules produce hormones outside the control of the pituitary gland. If they begin to function at a higher rate than they should, they can contribute to hyperthyroidism by overproducing thyroxine. Veterinarians administer hormone medication to dogs suffering from canine hypothyroidism, which is the opposite of hyperthyroidism and a common condition caused by the production of too little thyroxine. Over-response to the medication can cause a dog to develop hyperthyroidism instead.

Studies into the causes of hyperthyroidism in dogs show evidence that a diet based on large quantities of raw meat contributed to a dogs' contracting the illness. A well-balanced raw food diet is unlikely to cause harm, but a diet containing fresh or dried gullets typically contains thyroid tissue. The dogs studied showed high concentrations of the hormones found in thyroxine and signs of thyrotoxicosis, or toxicity caused by thyroxine.

When a veterinarian suspects a thyroid carcinoma, he usually begins by scanning the dog’s throat to identify the location and size of the tumor. This helps to determine whether the tumor is cancerous and whether it has spread to surrounding tissue and organs. If the results of the scan appear to show a tumor, the veterinarian confirms it by taking a biopsy or by performing surgery to remove it. The tumor is sent for analysis to determine whether it was malignant.

If the tumor appears to move freely in the neck, the veterinarian typically removes it surgically as the first step in the dog’s treatment. If the tumor is attached to tissue, however, the veterinarian might try radiation therapy instead. This avoids surgery, which could cause damage to nerves and blood vessels. He may also recommend chemotherapy to minimize the risk of the cancer spreading. In cases of dietary hyperthyroidism, the dogs studied showed signs of improvement when they were given a different diet.

Regardless of the treatment your veterinarian uses, malignant carcinomas that have metastasized are seldom curable. With ongoing treatment and palliative relief, however, your dog may live for several more years without the discomfort of a tumor in the throat or symptoms of hyperthyroidism.