Hyperthyroidism in cats (Proceedings)

Hyperthyroidism is a clinical condition resulting from the excessive production and secretion of thyroxine (T4) and triiodothyronine (T3) by the thyroid gland which was first reported around 1979. These hormones regulate the body's metabolic rate and affect every system in the body. The production of the thyroid hormones is controlled by the hormone called thyroid stimulating hormone (TSH). TSH is produced by the pituitary gland, which is found at the base of the brain. The thyroid gland is small and consists of two lobes, one on each side of the trachea (windpipe) in the neck. If the thyroid gland produces excess amounts of the thyroid hormones, the condition called hyperthyroidism results. The most common cause is a benign (non-cancerous) increase in the number of cells in the thyroid gland. Groups of these abnormal cells form small nodules (multinodular adenomatous hyperplasia or goiter)on the thyroid gland or adenomas. Multiple adenomas may form in the same lobe, and in approximately 70% of the cases, both lobes are involved. Only 1-2% of hyperthyroid conditions in cats are caused by malignancy (cancer). The incidence of hyperthyroidism in cats has increased remarkably in the last 25 years. The etiology is unknown, but probably due to multiple factors. The ingredients and types of foods fed, immunological factors, and environmental influences may be involved. Because of the rising incidence of this condition the need for veterinary nurses to be educated and able to provide good care to these patients is indeed important.

Signalment

It is common in middle-age to older cats with >95% of the cases occur in cats over 8 years of age. The actual age range is 4y-22y with the mean age being 13years old); so these are typically older cats. Only 5% of hyperthyroid cats develop the disease before 8 years of age. There does not appear to be a breed or sex predilection.

Signs commonly observed by owners of cats

Clinical signs

Thyroid hormone affects every organ system, so signs can be variable. The most common clinical signs include weight loss, increased appetite, vomiting, diarrhea, drinking and urinating more, and nervousness or hyperactivity. Thyroid hormone affects the heart, causing fast heart rates, heart murmurs, abnormal heart beats and high blood pressure. Many times owners will report that their cat appears "healthy", that they have a great appetite and are very active.

Breakdown of Documented Clinical Signs

Rapid heart rates are common in cats with hyperthyroidism, and heart murmurs and high blood pressure can also occur. Cats with hyperthyroidism that are not treated often develop hypertrophic cardiomyopathy, in which the muscle of the heart becomes excessively thick. This can lead to heart failure and death. It is common for these patients to be co-management by a cardiologist.

Screening criteria

There are three main criteria for diagnosing hyperthyroidism

1. Clinical signs as described above

2. Palpation of an enlarged thyroid gland - Normally, the thyroid gland in cats cannot be palpated. In most cats with hyperthyroidism, the gland becomes large enough to feel. Sometimes the gland becomes so large, it actually migrates or "sinks" into the chest cavity, and cannot be felt. There may also be instances where thyroid gland tissue is found in other areas of the neck and chest. This is called ectopic thyroid tissue.

3. Increased thyroid hormone levels - High T4 levels indicates hyperthyroidism in cats. Elevated T3 levels also indicate hyperthyroidism, however, in 25% of the hyperthyroid cats T3 is not elevated even though T4 is high. For this reason, the blood level of T4 is primarily used to diagnose hyperthyroidism. Sometimes, an animal with concurrent kidney, heart, or other debilitating disease may have hyperthyroidism but a normal or only slightly elevated T4. If an animal is suspected of having hyperthyroidism but has a normal blood test it is suggested that the animal be re-tested after the current disease is under proper medical management.

Since many of the signs of hyperthyroidism can also be found in other diseases such as diabetes mellitus, kidney failure, heart disease, or liver disease, other laboratory tests such as a CBC, serum chemistry, and urinalysis are generally performed to determine if these diseases are present. The test results will also influence which type of treatment would be most appropriate. Cats with hyperthyroidism may have slight increases in the number of red blood cells, increased liver enzymes, and increased BUN and creatinine, which measure kidney function.

Accepted Diagnostic Screening for Hyperthyroid Cats

Occasionally, veterinarians will use other tests to confirm their diagnosis of hyperthyroidism. These include the T3 suppression test, thyrotropin-releasing hormone stimulation test, measurement of free T4, and thyroid radionuclide uptake and imaging.

Treatment of hyperthyroidism

Currently, there are three widely accepted treatment options for hyperthyroidism in cats and a fourth option being explored. All have there advantages and disadvantages (as can be seen on the chart to the right).

Advantages and disadvantages of hyperthyroidism treatment

Antithyroid medication

The use of oral antithyroid medication is still a very popular choice for the treatment of hyperthyroidism in the cat. The drug that is used is methimazole (Tapazole). Tapazole is a human drug. It is given in a pill form, daily for the life of the cat. It can also be administered as a transdermal gel applied to the ear. It works by blocking the synthesis of thyroid hormone in the gland. Methimazole is often used as a first course of therapy for hyperthyroidism. This can help reverse some of the abnormal metabolic and heart conditions associated with hyperthyroidism. This will decrease the anesthetic risk if surgery will be performed. This also can allow for better assessment of the kidney function.

Surgery

Surgery to remove the abnormal thyroid lobe is called thyroidectomy. Thyroidectomy is a common treatment unless the risk of anesthesia in the cat is unacceptable, the thyroid gland is extremely large, thyroid tissue is present in the chest cavity, or thyroid cancer with metastasis is suspected. Since the parathyroid glands are surrounded by thyroid tissue, it may be difficult to visualize them during surgery. The parathyroid glands control the calcium and phosphorous levels in the body. If the parathyroid glands are inadvertently removed during surgery, life-threatening hypocalcemia (low blood calcium levels) can occur. Blood tests to monitor calcium levels are generally performed at least once daily for 5 to 7 days if both lobes of the thyroid gland are removed. Clinical signs of hypocalcemia typically develop within 72 hours of surgery, although signs may not develop for 5 to 7 days. Surgery can be an effective cure for this disease. It eliminates the need for daily administration of medications and the animal only has to be hospitalized for several days.

Radioactive iodine

Radioactive iodine therapy provides a simple, effective, and safe treatment for cats with hyperthyroidism. The majority of iodine an animal receives in the diet or through an injection becomes concentrated in the thyroid gland. When given radioactive iodine, the iodine becomes concentrated in the thyroid gland and kills the over-producing cells. The parathyroid glands are not damaged by this treatment. A hyperthyroid cat usually receives the radioactive iodine by means of an injection. The cat will need to remain hospitalized following the injection since the cat has too much radioactivity to safely be at home. The radioactivity is checked daily and when it reaches an acceptable level, usually in 2-7 days, the cat may go home. Radioiodine treatment is the preferred treatment for cats with both lobes of the thyroid gland affected, and those with thyroid tissue in inoperable locations. The results and safety of this therapy appear to be excellent.

Chemical ablation

A new treatment, called "ablation" is being investigated. With this treatment, ultrasound is used to identify the thyroid gland, which is then injected with a chemical or exposed to heat via high radiofrequency to kill the abnormal cells. Although the procedure requires anesthesia, the treatment takes less than 15 minutes.

Conclusion

Hyperthyroidism is the most common endocrine disorder that affects cats. It usually strikes middle to older age cats and has a wide range of signs. Diagnosis is relatively easy with a blood thyroid level check. Treatment is usually successful and properly treated cats can lead normal healthy lives.