Spring Kitty Crazies? Or Overactive Thyroid.

April 2007 - by Doctor Denise E. Roche, D.V.M.  

A previous article dealt with dogs and hypothyroidism.  This addresses cats and their thyroid disease - hyperthyroidism.  Hyperthyroidism is an overproduction of thyroid hormone that increases a cat’s overall metabolic rate.  It is most often caused by independent hyperfunctioning nodules in the thyroid gland.  Either one or both lobes of the thyroid gland may be affected.  It is a non-malignant (benign) change in the thyroid tissue. Less than 2 % of hyperthyroid cases in cats involve malignant thyroid gland tumors. The exact mechanism of hyperthyroidism remains a mystery.

Many organ systems are affected by this disease, especially the heart.  As the metabolism increases so does the heart rate.  The heart is stimulated to pump faster and more forcefully.  The heart muscle eventually enlarges to meet the increased demands placed on it. This is called hypertrophic cardiomyopathy.  High blood pressure can also result.  Fortunately both the heart muscle enlargement and hypertension can be reversed with treatment early in the course of the disease. Increased blood flow through the kidneys can mask hidden renal disease.  The gastrointestinal transient time is decreased and malabsorption can occur.  The cat’s musculoskeletal and nervous system can also be affected.
Hyperthyroidism is the most common hormonal disease of cats. 

The average age of onset is 12 years of age, but a hyperthyroid patient can range in age from 4 to 22 years old.  Male and female cats are equally affected and there does not appear to be a breed predilection.

The most common clinical sign is weight loss despite a good appetite.  In fact most hyperthyroid cats have an increased hunger.  Some patients will have a ravenous appetite and will become food thieves.  Despite the increased food intake the patient continues to lose weight because of the increased metabolism.  The weight loss can happen very quickly or gradually over time.  Often owners believe that the gradual weight loss is a natural aging process.  Affected cats will often drink more water and frequently urinate.  Vomiting, diarrhea, increased respiratory rate or abnormal breathing patterns may be noticed.  Some cats will exhibit excitability, hyperactivity and possibly aggression.  Most cats will have an unkempt haircoat, thickened nails and an overall poor body condition.  A veterinary exam often reveals muscle loss, increased heart rate and a heart murmur.  Occasionally a patient will have an abnormal heart rhythm. About half of all patients will have an enlarged thyroid gland that can be palpated.

Clinical signs of feline hyperthyroidism can mimic those of chronic renal failure, diabetes mellitus, chronic liver disease and gastrointestinal cancer. A blood test must be done to diagnose feline hyperthyroidism.  A total T4 (thyroxine hormone) level will be elevated in hyperthyroidism. In fact the T4 level is usually so elevated that there is no question as to the diagnosis.  Other blood chemistries should be performed to check for concurrent underlying disease.  Special attention should be paid to kidney function tests since treating hyperthyroidism will slow the heart rate and will in turn slow kidney blood flow.

There are three hyperthyroid treatment options – radioactive iodine, oral anti-thyroid medication and surgery.  Many factors are considered when choosing which therapy is most appropriate. Radioactive iodine therapy (I 131) is the gold standard of therapy.  Radioactive iodine is given by injection and it destroys all the abnormal thyroid tissue without putting other organs at risk.  A veterinary hospital licensed to administer radiation therapy must provide the treatment.  A hospital stay of one to two weeks may be needed as a patient detoxifies. Usually a single session is all that is required and reoccurrence of the disease is uncommon after radioactive iodine therapy. This can be an expensive option and many owners are hesitant to hospitalize their geriatric pet for an extended hospital stay, but once the therapy and the stay are completed the treatment is over.

An oral medication, methimazole (Tapazole), can control the effects of hyperthyroidism. Methimazole interferes with absorption of iodine and therefore inhibits the synthesis of thyroid hormone. Methimazole has no effect on the abnormal thyroid tissue; therefore the medication must be given daily for the remainder of the cat’s life. Obviously administering daily medication to a cat may not be an easy task.  The methimazole can be formulated into a topical, transdermal gel that can be applied to the inner aspect of the ear.  However this formulation is expensive and absorption can be inconsistent.  The medication side-effects include anorexia, vomiting, and blood cell abnormalities.  Often these troubling side-effects will occur within the first three months of therapy. Periodic blood tests must be done to monitor potential abnormalities and to regulate the dosage of methimazole required to control the disease.  Keep in mind that years of methimazole therapy and years of monitoring blood tests can potentially equal the cost of radioactive iodine therapy. Methimazole treatment is often reserved for cats in poor health, cats with suspected underlying kidney disease or cats of advanced age.

Surgery is the third treatment option available although it is rarely done. The abnormal thyroid lobe(s) are removed.  Often the patient is placed on methimazole for several weeks prior to surgery to stabilize the patient and hopefully minimize adverse anesthetic and surgical sequela.  A hyperthyroid cat with secondary cardiac disease is obviously an anesthetic risk.  Recurrence of the disease is possible after surgery if abnormal thyroid cells remain in the cat.  This can occur if abnormal thyroid tissue has migrated down the neck and it was not removed during the procedure.  Surgery has fallen out of favor because of the efficaciousness of radioactive iodine therapy.
No matter the therapy chosen, follow up blood tests will need to be done, usually within the first month, to check the response to therapy and to screen for underlying kidney disease. Once the hyperthyroidism is under control, the kidney blood flow will return back to its normal levels. If there was borderline kidney function to begin with, kidney function tests may rise to abnormal levels. Renal failure may result. If methimazole therapy unmasks kidney disease then the methimazole dose will be adjusted to balance out kidney perfusion and thyroid regulation. Patients undergoing radioactive iodine therapy will be screened extensively for underlying kidney disease prior to treatment so this adverse complication is usually not a concern with that therapy.  Overall the prognosis for hyperthyroidism is good if the disease is detected in its earlier stages and there are no underlying complications.  Screen for this disease in your beloved, older feline pet with annual blood tests.