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Feline Hyperthyroidism

Feline hyperthyroidism is a systemic disorder caused by excessive thyroid hormone. Thyroid hormone regulates metabolism, and therefore, affects almost every system in the body. Hyperthyroidism is the most common endocrine disorder diagnosed in cats over 8 years of age. The average age of onset is 12 to 13 years.

What are the Signs?
The signs of hyperthyroidism include weight loss, hyperactivity, increased appetite, increased thirst and urination, vomiting, diarrhea, heat intolerance, poor hair coat, increased heart rate, abnormal heart beat and/or heart murmur. Advanced cases may show signs such as anorexia, depression and weakness. These are general signs and only raise the suspicion of a thyroid disorder.

How is it diagnosed?
If after examining your cat the veterinarian finds the signs of hyperthyroidism a blood test to measure the level of thyroid hormone can be done. Other blood tests may be performed to evaluate other body systems. There is a link between hyperthyroidism, liver, kidney and heart problems. Therefore, careful monitoring of liver and kidney function is essential. Frequent examinations by your veterinarian are the best way to detect changes in your pet especially any problems with the heart. If your cat is not treated, it will continue to exhibit the signs of hyperthyroidism and eventually become sick and suffer. The disease will not resolve without treatment. With the proper veterinary treatment your cat can resume a normal life.
Treating Hyperthyroidism
Methimazole or Tapazole is the most common treatment. There are few side effects from this drug and patients are usually controlled with one tablet two times a day. Tapazole blocks the production of thyroid hormone but does not destroy the abnormal thyroid tissue. If the medication is discontinued the signs of hyperthyroidism will recur within 24 to 72 hours. The first few months of treatment require repeated blood tests to assure the proper dosage of medication is being given. After the correct dosage is achieved, your veterinarian may recommend regular blood tests to monitor the condition.
Radioactive Iodine
Radioactive iodine provides a simple, effective and safe treatment for cats with hyperthyroidism. The radioactive iodine concentrates in the thyroid and destroys the abnormal tissue. This may not be the best treatment option if there is also heart or kidney disease. Treatment involves an extended stay at the hospital. A single dose of the radioactive iodine usually restores normal thyroid hormone levels. Occasionally after treatment, hypothyroidism develops and thyroid supplementation is needed. Repeat blood testing is required after treatment. This treatment is only available at hospitals licensed to use radioactive medications.
Thyroidectomy or removal of the thyroid gland is an effective treatment for hyperthyroidism. There are potential complications such as nerve damage to the eye or voice box, low blood levels of calcium and damage to the parathyroid (the gland that is next to the thyroid gland). Surgery may not be an option for those patients who have heart conditions or other illnesses that prevent them from undergoing anesthesia. After surgery, there is a possibility for recurrence of hyperthyroidism; therefore, thyroid hormone levels should be monitored yearly after surgery. With the advent of radioactive iodine treatments, surgery is less likely to be recommended.
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