Hyperthyroidism in Cats

 Disease

Hyperthyroidism is caused by a tumor, usually benign, of the thyroid gland that secretes excessive amounts of thyroid hormone. This has many effects on the body since the thyroid controls many basic body functions such as heart rate and contraction, appetite, weight control, heat tolerance, water consumption, to name a few. When a cat is hyperthyroid, these functions become exaggerated.

Clinical signs

In most cases, they eat with exceptionally good appetites but lose weight. They drink and urinate more. Frequent vomiting (several times a month) and/or diarrhea often occur. Their heart beats faster, and the heart muscle can become thickened (hypertrophied), a state that can lead to heart failure. High blood pressure can occur, and hypertension can lead to further strain on the heart and eye problems (such as detached retinas). Behavior changes such as nervousness, restlessness or even aggression can result. If left untreated, this condition will result in a progressive weight loss and overly stressed organs.

Diagnosis

Cats with these signs are suspected of being hyperthyroid and sometimes an enlarged thyroid gland can be felt along the bottom part of the neck alongside the windpipe. However, the diagnosis is made on the basis of a blood test.

A complete panel and urinalysis are helpful to rule out concurrent diseases, and it is specifically a T4 level or sometimes, a free T4 (done by a different laboratory technique) that diagnoses this condition.

Assessment of the cat's blood pressure is recommended since hypertension is a common complication of hyperthyroidism. Sometimes, this resolves with the treatment of the hyperthyroid state. However, sometimes, it necessitates specific medication to control. As with humans, high blood pressure has several negative side-effects including additional stress on the heart and kidneys and the risk for sudden blindness due to retinal detachment.

Once your cat's condition has been diagnosed, your veterinarian will discuss treatment options. There are 3 treatments, each with advantages and disadvantages.

Medication

The most commonly used medication is methimazole (Tapazole). This can be prescribed through a human pharmacy as a tablet or a veterinary compounding pharmacy as a flavored chewable tablet or a flavored liquid, given orally, or as a transdermal form (a paste applied topically to inside the ear). Depending on the cat's starting T4 levels and the cat's response to the medication, it is given 1-2 times a day.

Side effects

Negative reactions to the medication can occasionally occur: decreased appetite (some leveling off from a ravenous appetite is normal after starting treatment), vomiting, diarrhea and itchiness of the head and neck. Adverse reactions, if they occur, usually happen in the first 3 months of treatment. Monitoring

After starting medication, at minimum, a blood test of T4 levels will need to be rechecked every 3-4 weeks until control is achieved. This is not a time tested and can be done at any time in relation to when the medication was given. It is ideal to do a complete blood panel that includes a T4 level since this allows for evaluation of the cat's other organs. Liver enzymes are often increased with hyperthyroidism; they should go back into the normal range with treatment of hyperthyroidism. When they remain increased or continue to go up, there is concern for concurrent liver disease. The kidney values are evaluated also since some cats will also have mild kidney failure that is "masked" by the hyperthyroid state. In these cases, cats will only "show" their kidney problems once the hyperthyroidism is treated. It also measures platelets and red blood cell counts that are sometimes decreased in numbers by the medication.

Control

Control of hyperthyroidism is achieved when the cat is doing well - no longer vomiting or having diarrhea, is no longer losing weight and is showing no ill effects from the medication and when the lab values are within normal. For most cats, this can include blood tests every 3-4 weeks until the proper dosage is attained.

Long term maintenance

For long term maintenance, a complete physical exam and complete blood panel including T4 and urinalysis and blood pressure measurement are recommended for every 6-12 months thereafter.

Pros of medication

It is relatively inexpensive. Depending on what formulation, how often the medication must be given and which pharmacy is used, costs can vary from $35-$60/month. Cons of medication

The cost of the medication will accumulate over the cat's lifetime since this is a long term treatment. A cat will not become "unhyperthyroid" and thus, is dependent on the Tapazole to suppress the overly active thyroid gland. If the medication is stopped, the cat becomes hyperthyroid again, and the disease process continues.

Blood tests need to be done at regular intervals to ensure continued control of the hyperthyroidism, and these laboratory costs can accumulate (depending on which lab tests are done, the cost can vary from $50-$100 each time). Frequency of testing depends on how the cat responds to the medication and presence of concurrent diseases.

Treatment with radioactive I 131

This treatment permanently treats the cat's condition by eliminating the hyperactive thyroid tissue. This radioisotope works specifically on the thyroid gland and doesn't affect the other organs. This treatment can be done at any time during a hyperthyroid cat's life, even after initiation of treatment with tapazole. Recent blood work and urinalysis is required before referral for this treatment. If a patient has been on tapazole for more than 90 days, stopping the drug for 7 days prior to I131 treatment is required.

Long term maintenance For long term maintenance, treated cats are recommended to have their T4 levels monitored the first and third months post treatment. A complete physical exam with a complete blood panel including T4 and urinalysis and blood pressure measurement are recommended for every 6-12 months thereafter.

Pros of I131

It is a permanent treatment, and it avoids the need for daily medication with Tapazole and eliminates the need for frequent lab testing. It is the treatment of choice for cats that are experiencing adverse effects to Tapazole.

Cons of I131

The treatment is done at only a few facilities with the necessary equipment (a referral can be made for hospitals in the area providing this service).

It can be costly. Total treatment costs are usually around $850-1000. This includes a scan using technetium (not done at all facilities), the I131 injection, general health and radiation monitoring and the stay at the hospital.

After treatment, since the cat and the cat's wastes (urine and feces) are radioactive, they need to be hospitalized for a period of time until their levels drop to an acceptable level. The usual stay in the hospital is 4-10 days during which the cat cannot be visited. Even after discharge, for a period of time-usually 2 weeks, there are certain precautions to be followed such as limiting close contact and extra care with removal of urine and feces from the litter box.

While usually only one treatment with I 131 is needed, occasionally, a second treatment is needed. This is needed in less than 2% of cats. Almost all cats are cured after a second treatment.

Also, while most of cats that have received I 131 will have normal thyroid function after treatment, there are a few cats that have too low thyroid levels (hypothyroid) after treatment. This can occur in less than 5% of cats. If these cats show signs of low energy levels and dull coats with dandruff, these cats will need thyroid supplementation. This medication, thyroxine (Soloxine), is a long term treatment. However, it is less expensive than Tapazole, and fewer medical problems arise from hypothyroidism compared to hyperthyroidism.

Surgery

Surgery can be done to remove the hyperthyroid gland (thyroidectomy). This approach is usually recommended in cases where the thyroid tumor is malignant. Long term maintenance

For long term maintenance, treated cats are recommended to have their calcium levels monitored daily the first 5-7 days post surgery. A complete physical exam with a complete blood panel including T4 and urinalysis and blood pressure measurement are recommended for every 6-12 months thereafter.

Pros of Surgery

It is usually a permanent treatment.

Cons of Surgery

These cats are often at risk with anesthesia. Occasionally, the hyperthyroid tissue can re-grow, resulting in recurrence of the hyperthyroidism, months to years after a successful thyroidectomy. Occasionally, hypothyroidism can result after surgical removal, thus necessitating thyroid supplementation.

Occasionally, inadvertently, the parathyroid glands, responsible for control of calcium and phosphorus levels of the body, can be removed during surgery to result in low blood calcium levels, a life-threatening problem that can result in seizures and muscle spasms. This condition is usually temporary, and calcium supplementation can help treat this, but calcium levels must be closely monitored post-operatively.

Ablation

This relatively new technique is done only at UC Davis School of Veterinary Medicine where ultrasound-guided ablation of the hyperthyroid tissue is done by an injection of ethanol. The technique is considered safe with relatively few side-effects.

Prognosis

Like most other diseases, hyperthyroidism is best diagnosed and treated in its early stages. The prognosis depends on the cat's condition at the time of diagnosis, its response to treatment and the presence of concurrent diseases such as kidney failure, diabetes, liver disease and cancer.

Whatever mode of treatment that is chosen to treat your cat's hyperthyroidism, it is important that you consistently give medication, follow up for necessary tests for monitoring your cat and notice changes in your cat's overall demeanor, energy levels, appetite, water consumption and urine output.

It is important you work closely with your veterinarian in following up with the appropriate tests and advice to ensure control of your cat's disease. Together, you can provide a good quality of life for your cat.

If you have any questions regarding this disease or its treatment, don't hesitate to contact your veterinarian.

The above is general veterinary information. Do not begin any course of treatment without consulting your regular veterinarian. All animals should be examined at least once every 12 months.

About the author:

Linda Mar Veterinary Hospital and its cat-only affiliate, Coastal Cat Clinic, are small animal practices located in Pacifica, California. To find a veterinarian or to learn more about the vet clinic and our staff, visit:[http://lindamarvet.com/]

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