The thyroid gland is one of the most important glands in the body of humans and pets. It is located in the neck near the trachea or windpipe and has two lobes, one lobe on each side of the trachea. The gland is controlled by the pituitary gland which is located at the base of the brain.
Basically, the thyroid gland regulates the body’s metabolism. Hyperthyroidism is when the thyroid gland is overactive and produces too much thyroid hormone. Hypothyroidism is an underactive thyroid gland which does not produce enough thyroid hormone.
Most commonly, dogs have problems with an underactive gland (hypothyroid) and cats have problems with an overactive gland (hyperthyroid). In dogs, hypothyroidism is usually caused by an immune-mediated disease. This means that their immune system decides that the thyroid is abnormal and attacks it. It is unclear exactly why this occurs. The normal thyroid tissue is replaced by fat tissue.
The most common signs of hypothyroidism in dogs are weight gain, lethargy, thin to nearly bald hair coat, failure to re-grow hair after clipping, cold intolerance and increase susceptibility to skin infections. Often, the dog will have elevated blood cholesterol. The thyroid plays a role in practically every part of the body. Therefore, other non-specific signs can also occur. Low thyroid can cause infertility, non-painful lameness, thickening of skin, and even decrease tear production in the eyes.
Thyroid testing is performed by collecting blood from the pet. There are different types of thyroid hormone that may be analyzed. Testing levels of T4, T3, Free T4 and TSH are performed to better determine if the patient is hypothyroid. If only one hormone is abnormal and there are no clinical signs present, the dog may be normal. Once your dog has been definitively diagnosed, treatment consists of oral administration of thyroid replacement hormone. This medication must be given for the rest of the dog’s life. After one month of treatment, further testing is done to verify that the hormone levels are normal. Typically, testing every six months is necessary to maintain proper hormone levels.
Cats have just the opposite problem with thyroid hormones. An overactive thyroid (hyperthyroidism) is a fairly common disease of older cats. Although the thyroid glands enlarge, it is usually benign or non-malignant change. Although environmental and dietary factors have been investigated as potential causes of overactive thyroid, the specific mechanisms are not known.
The most common symptoms of hyperthyroidism in cats are weight loss in conjunction with increased appetite. In fact, some cats have a ravenous appetite and will literally eat anything in sight. Despite their appetite, most hyperthyroid cats become very skinny. They may have some vomiting or diarrhea and their fur may appear unkempt.
Complications often occurs secondary to the hyperthyroidism. High blood pressure and heart disease are common complications. In some cats, their blood pressure can become so high that retinal hemorrhage in their eyes will occur resulting in blindness. Heart problems may develop due to the increased pressure and the heart enlarges trying to keep up with the increased metabolic rate. Usually, an obvious heart murmur can be detected by your veterinarian.
Fortunately, these problems are potentially reversible with appropriate treatment of the hyperthyroidism.
Just like in dogs, testing for this disease requires obtaining a blood sample to measure the thyroid hormone levels. Depending on the cat’s condition, other tests such as blood tests, urinalysis, EKG or ultrasound may be indicated to determine the cat’s overall health.
There are three basic ways to treat cats with overactive thyroid glands. First, radioactive iodine can be given by injection which destroys all abnormal thyroid tissue without endangering other organs. However, this kind of treatment requires one or two weeks of hospitalization at a veterinary hospital licensed to administer radiation therapy.
Secondly, surgery can be performed to remove the affected thyroid lobe. Risk of anesthesia must be considered since hyperthyroid cats are usually over eight years old, very underweight, and often have heart problems.
Additionally, oral medication can be used to treat hyperthyroidism. Methimazole is a medicine that can control the overactive thyroid. This medication is very effective by blocking the production of excess thyroid hormone rather than destroying the thyroid tissue. Cats managed medically usually do very well as long as the medication is administered routinely for the rest of the cat’s life.
Therefore, if your dog shows any signs of underactive thyroid (hypothyroid) or your cat shows any signs of overactive thyroid (hyperthyroid),contact your veterinarian as soon as possible to ensure your pet lives a long, healthy and happy life.
