At the Animal Clinic at Thorndale, we monitor our patient’s weight carefully over the years. For a cat, even a weight loss of 1 pound can be significant, considering it may be 10% or more of the total cat’s weight. Unexplained weight loss is always something that needs attention. Cats that are eating well but loosing weight puts hyperthyroidism on our potential problem list.
Weight Loss with a Good Appetite? Think Hyperthyroidism
The thyroid gland is located in the neck and plays a very important role in regulating the body’s rate of metabolism. Hyperthyroidism is a disorder characterized by the overproduction of thyroid hormone and a subsequent increase in the metabolic rate. This is a fairly common disease of older cats. Although the thyroid gland enlarges, it is usually a nonmalignant change (benign). Less than 2% of hyperthyroid cases involve a malignancy (cancer).
Many organs are affected by this disease, including the heart. The heart is stimulated to pump faster and more forcefully; eventually, the heart enlarges to meet these increased demands for blood flow. The increased pumping pressure leads to a greater output of blood and high blood pressure. About 80% of cats with hyperthyroidism have high blood pressure. Many have fast heart rates and even heart murmurs.
Contributing Factors
Advancing age is the main factor that increases a cat’s risk for hyperthyroidism. Environmental and dietary risk factors are currently being investigated and may play a role in predisposing cats to hyperthyroidism, though the specific mechanisms are not, as yet, known.
No individual breed is known to be at increased risk, although the Siamese appears to have a 10-fold lower risk of developing hyperthyroidism than other breeds.
Clinical Signs
The typical cat with hyperthyroidism is middle-aged or older; on the average, affected cats are about 12 years of age, though some may be diagnosed as early as six years of age. The most consistent finding with this disorder is a loss of weight secondary to the increased rate of metabolism. The cat tries to compensate for this with an increased appetite. In fact, some of these cats have a ravenous appetite and will literally eat anything in sight! Some cats are vomiting, usually after rapidly consuming a meal. Despite the increased intake of food, most cats gradually lose weight. The weight loss may be so gradual that some owners will not even realize it has occurred. Affected cats usually drink a lot of water and urinate a lot. Kidney problems are commonly co-existing in these cats. There may be periodic soft stool or diarrhea, and the hair coat may be unkempt. In some cats, anorexia (not eating) develops as the disease progresses.
Two secondary complications of this disease can be significant. These include hypertension (high blood pressure) and a heart disease called thyrotoxic cardiomyopathy. Hypertension develops as a consequence of the increased pumping pressure of the heart. In some cats, blood pressure can become so high that retinal hemorrhage or detachment will occur and result in sudden blindness. Typically these cats have dilated eyes and may bump into objects in the house. The heart problems develop because the heart must enlarge and thicken to meet the increased metabolic demands. Both of these problems may be reversible with appropriate treatment of the disease if caught in the early stages.
Causes
One potential consequence of treating hyperthyroidism is kidney disease or failure. In some geriatric cats,kidney function has declined with age. When hyperthyroidism is present, it helps the cat partially compensate for this loss of kidney function. The hypertension that accompanies hyperthyroidism serves to increase blood flow to the kidneys. When the hyperthyroid state is treated, renal blood flow is diminished and the cat may develop kidney failure. This occurs in only a small number of hyperthyroid cats and is most likely if kidney function tests before treatment are abnormal.
Diagnosis
In most instances, diagnosis of this disease is relatively straightforward. A thorough physical examination and weight check, complete blood count and blood chemistry, and urinalysis are needed to check for other metabolic disorders which may affect treatment options. One of the first things to happen in hyperthyroidism is an increase in size of one or both thyroid lobes. If they can be palpated (felt) during the physical exam, the disease is very likely. Thyroid level blood tests, T4 and Free T4 by equilibrium dialysis, confirm the actual diagnosis. Usually, the T4 level is so high that there is no question as to the diagnosis. Occasionally, a cat suspected of having hyperthyroidism will have T4 levels within the upper range of normal cats. These cats need to be closely followed for changing thyroid levels.
Treatment Options
Because less than 2% of these cats have cancerous growths of the thyroid gland, treatment is usually very successful. There are three choices for treatment: radioactive iodine, methimazole, and surgery. Many factors must come into consideration when choosing the best therapy for an individual cat.
When possible, tests are done before adopting any form of treatment. These tests are needed to evaluate the overall health of the cat and predict the chances for complications. Such tests include blood work, a urinalysis, and chest x-rays. An EKG (electrocardiogram) and cardiac ultrasound may be performed. If blindness is present, an ophthalmology consult may be warranted.
- Radioactive iodine. The most effective way to destroy all of the abnormal tissue is with radioactive iodine therapy. It causes no damage to normal thyroid tissue or to the nearby parathyroid gland. This requires three or more days of hospitalization at a veterinary clinic licensed to administer radiation therapy. This treatment is limited to specialty practices because of governmental regulations regarding radioactive materials. We are fortunate to have area hospitals which perform this procedure. To read more about this option go to http://www.radiocat.com . Radioactive iodine is generally curative. Follow up consists of periodically measuring a baseline thyroid level.
- Oral medication. Administration of an oral drug, methimazole, can control the effects of the overactive thyroid gland. Some cats have reactions to the drug, but that number is fairly small (less than 20%). However, the side effects may begin as late as six months after the beginning of treatment and can include vomiting, lethargy, anorexia, fever, and anemia. Methimazole does not destroy the abnormal thyroid tissue, but rather ties up the excess thyroid hormone. Therefore, the drug must be given for the remainder of the cat’s life, usually twice a day. Periodic blood tests must be done to keep the dosage regulated. This type of treatment is appropriate for the cat that is a poor candidate for radioactive iodine treatment due to co-existing kidney or heart problems or when the up front cost of iodine therapy is prohibitive. It is important for owners to accept that methimazole therapy requires a life long commitment. An abrupt cessation of treatment can result in a “thyroid storm” in which rapidly increasing thyroid levels can have devastating effects.
- Surgery. Surgical removal of the affected thyroid lobe was performed often in the past. It can be effective, but it is currently not recommended in most cases. Surgery can be very invasive and carries significant risks. Today, surgery is usually reserved for a suspected cancerous thyroid gland.
Recurrence of the disease is a possibility in some cats. Recurrence is uncommon after radioactive iodine therapy. When surgery is done, it is possible. It is possible that not all of the abnormal thyroid cells will be removed. With methimazole treatment, thyroid levels will rebound quickly when treatment is stopped.
One potential consequence of treating hyperthyroidism is the unmasking of kidney disease or failure. In some geriatric cats, kidney function has declined with age. When hyperthyroidism is present, it helps the cat partially compensate for this loss of kidney function. The hypertension that accompanies hyperthyroidism serves to increase blood flow to the kidneys. When the hyperthyroid state is treated, renal blood flow is diminished and the cat may develop kidney failure. This occurs in only a small number of hyperthyroid cats and is most likely if kidney function tests before surgery are abnormal. In cats where it is known that kidney function is compromised, radioactive iodine treatment is contraindicated. These cats can usually be well controlled by adjusting methimazole treatment against the changing kidney function.
Prognosis
Many owners of cats with hyperthyroidism are hesitant to have radiation therapy because of their cat’s advanced age. But remember, old age is not a disease. The outcome following radiation therapy is usually very positive, and most cats have a very good chance of returning to an excellent state of health for many years. Cats treated with methimazole generally also do very well. Occasionally the medication needs to be temporarily stopped and restarted at a lower dose if digestive problems occur. Even these cats can be brought back into the therapeutic range by slowly adjusting the dose upward.
Prevention
There are no known preventive measures, but middle-aged and geriatric cats should all receive a complete physical examination by a veterinarian every 6-12 months. If owners notice significant weight changes in their cat, it should be brought to the attention of their veterinarian. A specific cause for hyperthyroidism has not been identified. The possible role of dietary iodine continues to be investigated as a dietary influence on the development of hyperthyroidism.