Feline hyperthyroidism was first recognized as a distinct clinical entity in 1979. Since then, it has become an extremely important and common disorder of older cats. The syndrome is a result of excessive circulation of the active thyroid hormones thyroxine (T4) and triiodothyronine (T3).
The cause of this increase in hormones in 95-98% of cats is due to a benign tumor (adenoma) of the thyroid gland. The other 2-5% of thyroid hormone elevations are due to thyroid carcinoma. Due to the fact that most cats are affected by the benign tumor, a favorable prognosis can be given for management, especially if diagnosed early in the disease.
Hyperthyroidism can present with a multitude of other problems due to the changes that the increased thyroid levels cause. Many cats present with an increased appetite, weight loss, hypertension (high blood pressure), cardiac abnormalities, vomiting and diarrhea, unkempt haircoat, increased water consumption and possible kidney disease,. Fortunately, as routine evaluation for disease increases in our patients, secondary problems of hyperthyroidism can be managed (or essentially eliminated) due to early diagnosis and treatment.
If hyperthyroidism is suspected, your veterinarian
will run blood tests
to evaluate the rest of the system. An elevation of the T4 levels
is diagnostic for hyperthyroidism. In suspect cats with T4 levels in the high normal range, a T3 suppression test or a Free T4 by Equilibrium Dialysis can be performed. In areas where thyroid scans are available, a scan can be performed to confirm diagnosis in questionable or early stage disease. Additional tests
to evaluate the heart
may be ordered and may include chest and/or abdominal radiographs, electrocardiograph (ECG), an ultrasound and an urinalysis.
Treatment of hyperthyroidism involves managing circulating circulating thyroid levels and/or eliminating the abnormal thyroid tissue. Prior to treatment, most facilities recommend placing the cat on methimazole for 2-3 weeks and re-evaluating the blood work and urinalysis. This check is to evaluate for the cat’s response to therapy and to evaluate if there is any underlying kidney disease that was “covered up” by protective effects of the body to the circulating thyroid levels. Once the results are obtained, a decision will be made to continue controlling levels with medication (similar to controlling insulin levels in diabetes) or to proceed forward with eliminating the abnormal tissue.
Abnormal tissue elimination can be obtained with radioactive iodide (I131)treatment at a special facility or with surgical removal of the thyroid gland. Both methods have their advantages and disadvantages, however, in a healthy cat, the outcome is usually excellent and most cats have a very good chance of returning to a normal state of health.
It should be noted that whatever method of treatment/control is chosen, the cat should have regular veterinary evaluations
and laboratory monitoring based on age and other concurrent issues. Additionally, many referral centers for I 131 therapy have different pre-treatment requirements. Your veterinarian will be familiar with the specifics when preparing your cat for therapy.
Many are available with similar information. They all provide images of scans and detailed information regarding the various treatment options.
Washington State University has an excellent page provides further information, especially related to thyroid scans: Hyperthyroidism in the Cat: Pet Health Topics
Gulf Coast Veterinary Specialists provides another look and aspect to thyroid disease:Feline Hyperthyroidism
Advanced Veterinary Medical Imaging provides similar information but includes pictures to highlight what is happening in the thyroid gland:Feline Hyperthyroidism
U.S. Pharmacist, a jobson publication, has an excellent article which details the differences between dog, cat and human thyroid disease.Thyroid Disease in the Cat and the Dog
Kari Mundschenk, DVM
Just For Cats Veterinary Hospital
Elk Grove, California
5 September 2007