Hyperthyroidism in Cats

Hyperthyroidism (syn.: Thyrotoxicosis) is a condition where the thyroid glands produce too much thyroid hormone, usually due to a benign thyroid gland tumor, and generally affects middle-to-older-aged cats. Since the thyroid hormone controls the body’s metabolism, an overproduction of this hormone causes an increase in the body's metabolic activity and often results in some or all of the following symptoms: increased appetite, thirst and urination, diarrhea, weight loss, fast or difficult breathing, heart problems, and irritability or hyperactivity. Very rarely (1-2%), the enlarged gland is affected by a malignant tumor growth. Almost 3/4 of patients have both glands involved.

Some animals actually decrease their food intake and become quieter (apathetic), however this is an uncommon manifestation of this syndrome. As the symptoms mimic a variety of other potential conditions, it is important from a diagnostic point of view for your veterinarian to take a blood and urine sample for evaluation. This is also done to monitor response to therapy. This is a serious condition because it can have detrimental effects on a variety of other body systems.

Diagnosis is made by performing a physical examination, obtaining a history from home, assessing blood levels of thyroid hormones, taking X-rays or performing echocardiography, and if results are equivocal, special suppression and stimulation hormone tests are carried out.

Treatment is designed to control or eliminate the thyroid gland's overproduction of thyroid hormone. Treatment options include medication (e.g., methimazole) given orally once to three times per day, surgical removal of the thyroid gland(s), or radioactive iodine therapy. There are advantages and disadvantages for each of the treatment options.

Tablet medication is often the initial treatment choice, regardless of what long-term treatment option is chosen, with the medication starting at a lower dose and gradually increased based on your pet's response to therapy. Repeat blood and urine testing is used to define your pet's response and to ensure that any concurrent conditions (e.g., kidney disease, anemia) or reactions to the medication (low white blood cells etc.) do not complicate therapy.

Concurrent kidney disease is one of the most challenging aspects of case management in cats because as hyperthyroidism is controlled, the flushing of the kidney is reduced, potentially exacerbating any malfunction. Frequently, these cases require additional management. Recognition of this situation means that veterinarians now monitor urinary system function very closely in patients starting any therapy for hyperthyroidism, or perhaps not (fully) controlling the condition if pre-existing kidney problems are identified. For those that respond well to initial therapy, the tablet medication can be continued indefinitely, or definitive treatments such as surgery or radioactive iodine can be considered. In some cases, cats are not keen on taking pills every day. Formulating the medicine in a tasty liquid is another option for these finicky felines!
Note that the oral medication does NOT fix the problem, and the adenoma continues to pump out excess hormones. This can result in loss of control as the adenoma continues to grow.

Surgery for patients without concurrent uncontrolled medical conditions still remains a very popular choice. The surgery can be done unilaterally (remove a single, enlarged gland) or bilaterally (both glands removed) either sequentially or together. The most common reason for not doing surgical treatment is because of cardiomyopathy, or very high, uncontrolled thyroid hormone levels. Frequently, the patient is placed on oral hormone and heart medications and monitored closely. In many cats, the heart condition that leads to tachycardia (high heart beat), murmurs, and other abnormalities will stabilize once the thyroid hormones have returned to the normal range for a while. At that point, your veterinarian may prescribe surgical treatment. After surgery, if the parathyroid glands (little glands that sit on the thyroid gland) are not maintained or re-implanted after surgery, supplemental calcium may be needed. This is something the surgeon will be monitoring for.

Radioactive iodine is also highly effective, though much less frequently done. Facilities that perform this therapy have become more widely available, but a referral is always needed since the facility needs to be specially licensed to provide this type of therapy. It used to be routine that a 2-4 week stay in a facility was required. Now, some cases require lower dose therapy, and so stays can be shorter. This is considered the gold standard for therapy of hyperthyroidism in cats. Recurrence is least likely with this therapy.

To find out more about management of hyperthyroidism for your cat, see your veterinarian.