Hyperthyroidism is a very common disorder in middle to older age cats; the average age being 13 years old. It is in fact the most common hormonal disorder that affects cats. Hyperthyroidism is the condition where the thyroid gland produces excessive amounts of the thyroid hormones. These hormones primarily affect the metabolic rate. The most common cause is a benign increase in the number of cells of the gland. They form small nodules on the gland and usually both lobes are involved.
Diagnosis is made by clinical signs, palpation of the thyroid gland and hormone levels in the blood.
The most common clinical signs include: weight loss even with an increased appetite, vomiting and diarrhea, increased water consumption and urination, increase in activity and elevated heart rate, unkempt hair coat, irritability and restlessness.
Less common signs include tremors, weakness, panting, laboured breathing, decreased activity and loss of appetite.
Since many other diseases such as diabetes mellitus, kidney failure heart and liver disease, have some of the same symptoms, other tests are usually run, to rule out other problems.
Palpation of an enlarged thyroid gland: The thyroid gland in cats can’t normally be palpated as it is a flat shape, on either side of the trachea, but in cats that are hyperthyroid; the gland becomes enlarged and can be felt.
Sometimes the gland becomes so big and heavy that it sinks down toward the chest cavity and can’t be palpated by the veterinarian.
Increased thyroid hormone levels: Diagnosis is made by a blood test which measures the level of T4 (thyroxine) a thyroid hormone. A blood sample is drawn from the cat and high levels of T4 indicate hyperthyroidism. Elevated T3 (triiodothyronine) levels also indicate hyperthyroidism, but T3 levels are not elevated in all cats.
If the diagnosis is not obvious by blood tests, a nuclear medicine scan of the thyroid glands can be performed at specialty veterinary clinics. The cat is given a small dose of a radioactive compound that travels by the blood to the glands. Hyperactive thyroid glands accumulate more of the compound than normal glands and can be seen on images taken of the cat.
Treatment is directed at controlling the amount of excessive thyroid hormones secreted and can involve a variety of different approaches.
Oral medication given daily is the most popular choice for treatment.
It is lifelong therapy with a human drug called Methimazole (Tapazole). The pills can also be easily formulated into flavoured pills (beef and chicken flavour). The advantages are that medication is readily available and fairly inexpensive.
The disadvantages are that this is not a cure, it is lifelong therapy where medication is given twice daily and it can be difficult for owners to administer. Regular periodic blood tests are required to monitor levels of the hormones and sometimes medication dosages need to be altered. Methimazole is also often used as a first course of therapies. It can help reverse some of the abnormal metabolic and heart conditions associated with hyperthyroidism. This will decrease the anesthetic risk if surgery is going to be performed. As hyperthyroidism can mask kidney disease, this also allows for better assessment of the kidney function.
Surgical removal of the abnormal thyroid lobes; this is a good option as it cures the condition unless all of the abnormal tissue is not removed. Also the anesthetic risk needs to be assessed. An ultrasound scan can be performed to identify the location of the abnormal tissue and see whether both lobes are involved. It does cost more initially, but there are no lifelong medications and blood tests. The disadvantages include that the cat requires anesthesia, therefore must be a good surgical candidate and there are occasional post-op complications. Because the parathyroid glands are surrounded by the thyroid tissue and difficult to visualize, if the parathyroid glands are accidentally removed, therefore calcium and phosphorous levels of the body would have to be monitored. Other complication include paralysis of the larynx, hypothyroidism, and if some thyroid tissue remains in the cat, they will still be hyperthyroid.
Radioactive iodine therapy is simple, effective and safe. When the cat is given radioactive iodine by injection, the iodine concentrates in the thyroid gland and kills the over producing cells. This is an ideal treatment for cats with both lobes affected or when the thyroid tissue is located in an area not easily accessible during surgery, Advantages: no general anesthetic or surgery, no daily meds, and normal thyroid function resumes within a month.
It is an expensive procedure and must be done a specialty hospital. Rarely the procedure needs to be repeated and rarely causes hypothyroidism.
So even though hyperthyroidism is a very common, serious problem in older cats, it is manageable, and there are options for the owner. If your cat has any symptoms, see your veterinarian.