Thyroid

The thyroid gland is the body’s “speed control mechanism.” When it is set on slow (hypothyroidism) hair falls out and doesn’t grow back, the energy level is reduced, the heart rate slows and there is increased sensitivity to cold. The dog (this condition is rare in cats) usually gains weight with minimal food intake. Breeding animals lose interest or become infertile. Delayed immune responses allow skin, urinary, or respiratory infections. Sluggish nerve/muscle function can mimic hip or back problems. No one dog will have all these symptoms and some are quite rare. However, the condition appears to be very common.

It would seem that diagnosing hypothyroidism would be a simple matter of measuring the level of thyroid hormones but it’s not. There are two hormones to measure in several different states or by different methods. There are antibodies against both and some methods of measuring confuse hormone with antibody resulting in false elevations. It is also possible to measure thyroid stimulating hormone (TSH). Different labs use different tests so results from one lab cannot be compared against another.

Many illnesses and medications can temporarily lower thyroid hormones while the gland itself is normal. Probably the most accurate diagnosis is based on a combination of typical signs, low to extremely low thyroid hormones and high TSH. In many cases if the blood work does not rule out hypothyroidism treatment will be started to see if the symptoms resolve. Once medication is started periodic monitoring is indicated to make sure appropriate levels of thyroid hormone are maintained. Giving thyroid supplements to a dog with a normal thyroid can damage the gland.

At the other end of the spectrum hyperthyroidism speeds up the body. This condition has become increasingly common in elderly cats. Symptoms are usually weight loss in spite of a good appetite but some cats quit eating and others suffer from frequent vomiting and/or diarrhea. There may be a very rapid heart rate known as a gallop rhythm. Diagnosis is usually easy—an elevated T4, one of the thyroid hormones, is almost always present. If there are typical symptoms but the T4 is normal additional tests may be needed.

There are three methods of treatment available. One is surgery to remove the affected gland. The surgery itself is not difficult but the age and health of the patient certainly increases the risk. In addition, there are some cases where the glands are not located in the usual position and surgery may be attempted without success. Since the non affected gland may develop the same condition a second surgery may be necessary later. Also the para thyroid glands are closely attached to the thyroids and their damage or removal can cause a sudden life threatening drop in blood calcium.

Another treatment option is a tablet which is given two or three times a day and suppresses thyroid function for a variable period of time—sometimes as much as a year. Methimazole can cause side effects such as vomiting and anemia. Regular monitoring for thyroid levels and blood count should be done for the duration of time the cat is on the drug. Some cats may be put on methimazole temporarily to improve health before surgery or until the third option is available. The cost of monitoring and the difficulties of giving most cats a pill two or three times a day are the major drawbacks of this treatment. Another problem is that eventually the drug will become ineffective.

The third treatment option is radioactive iodine. This highly effective therapy is very safe. It does not require anesthesia or repeated treatments and monitoring. It is one simple intravenous injection. Because the radioactive iodine concentrates in the gland it is destroyed no matter where it is situated in the body. However it does not damage any surrounding tissue so the parathyroid is spared. The cat must remain in the hospital for 5-7 days while the radioactiviy fades and special precautions must be taken for a few weeks after the cat returns home.

This treatment is only available in a few locations and so it may be necessary to wait a month or more for a turn. The cost is a little over $1000. With effective treatment thyroid hormone levels reach almost to zero but there does not appear to be any problem with hypothyroidism in these cats. If the condition has been present for a good while there may be permanent damage to the heart and/or kidneys so it is important to evaluate these organs before treatment. Impending kidney failure may be a reason to decline treatment as it will get worse when the thyroid levels drop.