What is Hyperthyroidism?

The most common endocrine disease in cats and one of the most common diseases of late middle- aged and older cats. Hyperthyroidism, also known as "hyperthyroid disease," occurs when the thyroid gland enlarges, and starts producing excess amounts of thyroid hormone (thyrotoxicosis). This anomaly is usually caused by a benign tumor on one or both of the thyroid gland's lobes. Although thyroid tumors can be cancerous, the chances are only 1% to 2% of malignancy.

What Are the Symptoms of Hyperthyroidism?

• Increased Appetite
• Unexplained Weight Loss and loss of muscle mass
• Irritability or Nervousness
• Frequent Vomiting
• Unkempt-looking Coat
• Diarrhea
• Excessive Thirst
• Excessive Urination
• Weakness
• Lethargy
• Increased heart rate
• High Blood Pressure

Not all symptoms will be seen in every cat with hyperthyroidism, but any one or two of them should be a strong indication that testing is indicated.

How is Hyperthyroidism Diagnosed?

• Physical Exam: Palpation of your cat's neck area, the enlarged gland(s) may be felt. Heart rate and blood pressure will also be checked.
• Complete Blood Panel: Since hyperthyroidism can mimic the symptoms of other diseases, such as chronic renal failure or diabetes, a blood panel will usually show the "big picture," when combined with a thyroid-specific test, usually the T4, which will show thyroid levels.

Elevated levels of T4 will usually strongly indicate the presence of hyperthyroidism, although some older cats may fall within the "normal" range, and still have the disease. When in doubt, other thyroid screening tests will be used.

As hyperthyroid disease can predispose to other conditions, such as hypertrophic cardiomyopathy or hypertension, the blood panel will also reveal pertinent information on the heart and other organs, which is essential to know prior to choosing an ultimate treatment.

1- Normal Thyroid Gland
2- Normal Parathyroid
3- Normal Parathyroid
4- Enlarged Thyroid Gland

What Are the Treatment Options?

1. Anti-Thyroid Medication
   This is almost always the first treatment. The drug given is methimazole, which controls the production of thyroid hormones. Many cats do quite well on methimazole, but it will have to be given twice every day for life, with regular tests to monitor hormone levels, so dosage can be regulated. It may be the least expensive treatment option, depending on the cat's age at onset. The overall expense of medications and blood tests is $350 - $600 per year, for the rest of your cat's life.
   Advantages:
   • Non-Invasive
   • Relatively inexpensive, initially
   • The only treatment option for cats with kidney disease or hypertrophic cardiomyopathy. Disadvantages:
   • Side effects occur in some cats, including vomiting, appetite loss, fever, anemia, and lethargy. More serious side effects from long-term use include liver damage and bone marrow suppression, although these are not often seen.
   • Frequent veterinary follow-up is needed as dosing will need adjusting.
   • The medication does not affect the tumor size, but will keep the signs of disease in remission, as long as it is given.
   • Some cats (and/or owners) cannot handle twice-daily pilling, and the attendant stress can exacerbate other physical problems.
   
2. Surgery Surgery, called a thyroidectomy, is an effective treatment. Because hypertrophic cardiomyopathy is sometime present, a full workup is required to eliminate this potentially dangerous condition. Also, the patient must continue on anti-thyroid medication for 15 days prior to the surgery, so that kidney function can be retested. Advantages:
   • Outpatient procedure
   • Usually Curative
   • Eliminates the need for long-term medication
   Disadvantages:
   • Damage to the adjacent parathyroid gland.
   • If only one lobe is removed because the second appeared normal, a recurrence is a possibility.

   The total cost for oral anti-thyroid medication and regular blood work for 12-18 months will approximate the cost of surgery.

3. Radioiodine Treatment A single injection of radioactive iodine (R¹³¹) is given subcutaneously. The substance "finds" and destroys all diseased tissue, including any ectopic thyroid cells (those that have moved to other areas of the body) without harming any normal tissue. Because the cat is radioactive, he must remain in the veterinary hospital for five to thirty days until his radioactive levels are acceptable. Caregivers may be able to visit during that time, depending on the facility, but will only be able to view their kitty through a special leaded window. Typical diagnostic tests given prior to the treatment are:
   • Physical exam
   • Complete blood count
   • Chemistry panel
   • Thyroid panel
   • Thoracic radiographs

   The cat is also given the anti-thyroid medication for 15 days prior to treatment with R¹³¹. As with the surgical option, a cat with hypertrophic cardiomyopathy, kidney disease, diabetes, or any other serious condition is not a candidate for radioactive iodine therapy. Advantages:
   • Safe
   • No serious side effects
   • Usually Curative (2% of cats need second treatment)
   Disadvantages:
   • Expensive
   • The cat must be in otherwise good health prior to treatment.
   • Long hospital stay

   Since hyperthyroidism induces increases in blood pressure and blood supply to the kidneys, treating the disease will result in a drop in the blood supply to the kidneys. In a cat with kidney failure, this can cause a worsening of their kidney function in the few months after treatment for hyperthyroidism with either radioactive iodine or surgical removal of the gland. For this reason, patients with known kidney disease are often treated with anti-thyroid medications rather than surgery or radioactive iodine in an effort to preserve their remaining kidney function.

   In general, the treatment a cat receives for hyperthyroidism will depend on individual status, including heart and kidney function. Concern about kidney failure is a major determinant of the course of treatment and may eliminate radioactive iodine or surgery as an option. The advent of new kidney function testing makes it possible to assess each patient's risk of kidney failure following treatment for hyperthyroidism.
   

Expected Course and Prognosis

The prognosis for uncomplicated disease is excellent. Recurrence is possible and is most commonly associated with poor owner compliance with medical management. Re-growth of hyperthyroid tissue is possible but uncommon after surgical thyroidectomy and radioiodine treatment.