Feline Hyperthyroidism
Hyperthyroidism is a very common hormonal disorder seen in middle-aged to older cats. The disorder occurs when the thyroid glands begin producing excessive amounts of thyroid hormones. This happens most often due to the development of a benign thyroid adenoma (tumor). It is not known what causes this tumor to develop in certain cats.
The thyroid glands are paired glands located in the neck which secrete thyroid hormone. Thyroid hormone is important in controlling the metabolic rate of an animal. Cats with hyperthyroidism have an increased metabolic rate, so they tend to use up excessive amounts of energy within their bodies. This means that your cat may be eating more and always hungry, but still seems to be losing weight. In fact, this is one of the most common clinical signs of hyperthyroidism. Some other common clinical signs may include a patchy haircoat and hyperactivity. The onset of this disease is usually very slow, and signs are usually not recognized until they are severe. Sometimes, the hyperthyroid state can exist for months to years before if becomes severe enough for the owners to take note of the clinical signs.
Often, your veterinarian can feel a nodule in your cat's neck where the thyroid gland is located. This is usually a good indication that your cat has hyperthyroidism. The diagnosis can be confirmed by testing the T4 (thyroid hormone) levels in your cat's blood.
If your cat is diagnosed with hyperthyroidism there are three treatment options: medical treatment, surgical removal of the thyroid, or radioactive iodine treatments.
Medical management consists of the administration of anti-thyroid hormone drugs. This does not cure the disease, but does provide for the long term control of the disorder. These drugs will probably have to be administered twice daily for the life of the cat.
Surgical thyroidectomy is the removal of the affected parts of the thyroid glands. This can result in a permanent cure, but there can be complications. As with any surgery, there is always a risk with anesthesia. Hyperthyroidism can recur if some of the remaining thyroid tissue becomes hyperactive. A more common secondary complication of surgery is accidental removal of part or all of the parathyroid glands (small glands sitting adjacent to the thyroid glands) resulting in a hypoparathyroid state. This can result in transient hypocalcemia that may be need to be treated for a few weeks or months.
Radioactive iodine is the final treatment option. This works because the thyroid gland takes up the iodine (no other body tissue does this) and the iodine kills all of the affected thyroid tissue. All other body tissues are spared. This is a curative treatment and does not have any major side effects for the cat. However, it must be performed in a facility licensed to handle radioactive substances (usually a university teaching hospital), and an extended hospital stay for your cat is usually required. Most of the excess radioactive iodine is eliminated in the cat's urine, so the cat must remain in isolation until most of the radioactivity has been eliminated.