Recognizing and Treating Feline Hyperthyroidism
What is hyperthyroidism?
Feline hyperthyroidism is a common disease that affects middle age and senior cats (usually age 8 and older). Hyperthyroidism is a disorder caused by an enlarged thyroid gland which produces too much thyroid hormone (called T3 and T4). An enlarged thyroid is typically due to a tumor affecting one or both lobes of the gland. In the majority of cases, these tumors are benign; however, in 2-5% of cases, they are cancerous.
What are the symptoms?
Thyroid hormone affects numerous systems in the body (metabolism, heart, kidneys, and liver), so an unstable thyroid can have disastrous and even fatal consequences. Symptoms of an overactive thyroid include:
- Excessive thirst and urination
- Increased appetite
- Aggressive behavior (or overly energetic)
- Yowling and panting
- Weight loss (despite increased appetite)
- Muscle weakness
- Unkempt-looking coat or hair loss
The majority of cats with this health issue lose weight while simultaneously having an increased appetite and onset is typically between 12-13 years of age. Note: Some cats may demonstrate other symptoms not listed above (such as anorexia). It was once thought that these cats exhibited atypical hyperthyroidism. However, it is more widely thought that these animals have other problems in addition to the hyperthyroidism, such as kidney disease or inflammatory bowel disease.
How is hyperthyroidism diagnosed?
A veterinarian will perform a physical exam and a CBC (complete blood count) with a special, thyroid-specific test known as the T4 panel. During the physical exam, the veterinarian will feel the cats neck area (where the thyroid is located) and may be able to feel the enlarged gland. The cats heart rate and blood pressure will also be checked. Cats with hyperthyroidism will have an elevated heart rate and high blood pressure.
The results from the blood test will offer a more complete picture and also rule out other conditions, which can present similar to hyperthyroidism such as chronic renal failure or liver disease.
What is the treatment?
There are three basic methods of treating hyperthyroidism: drug therapy, radioactive iodine therapy, and surgery.
A veterinarian will prescribe one of two medicines usually taken twice daily: tapazole or methimazole. Both medicines are effective in regulating thyroid hormone levels (but do not destroy the tumor) and can be administered in pill form or as a transdermal cream that is rubbed in the ear. The advantages of antithyroid drug therapy are that it is a non-invasive, relatively inexpensive treatment. Medication is also the only treatment for cats with hyperthyroidism and underlying kidney disease. Hyperthyroidism tends to mask kidney disease, so a careful balance is required to ensure that thyroid levels return to an acceptable level without leading to renal failure. There are some cats, however, that experience side effects to the medication such as nausea, vomiting, fever, anemia, and decreased appetite. Also, the medication will need to be administered for the rest of the cats life, which causes a large amount of stress on both animal and owner.
Radioactive Iodine Therapy (R131)
This treatment is permanently effective in up to 95% of cats with hyperthyroidism and no underlying kidney disease. Radioactive iodine is injected under the skin. The thyroid gland metabolizes the iodine, but only the diseased cells of the thyroid gland are destroyed, leaving healthy tissue unaffected. This treatment is performed at a certified veterinary hospital and requires that the cat is hospitalized for 1-2 weeks until its radioactive levels are acceptable. Cats with hypertrophic cardiomyopathy, kidney disease, diabetes, or other serious condition are not candidates for radioactive iodine therapy. The advantages of this procedure are that provides a permanent cure, is safe with no serious side effects, and provides the lowest level of stress on the animal. However, this treatment is expensive and requires that the cat is in good health before treatment.
In some cases, the veterinarian may opt to surgically remove one of the lobes of the thyroid gland, known as a thyroidectomy. This treatment will also permanently cure hyperthyroidism if performed correctly. This is an expensive procedure that carries some risk. In some cases, not all of the damaged thyroid gland is removed, or, damaged is caused to the surrounding tissue in the throat. (Some owners have reported a change in the cats meow.) As with radio iodine treatment, not all cats are candidates for surgery.
What is the prognosis?
Cats with hyperthyroidism can lead healthy, happy lives provided that they receive proper, ongoing treatment.