Winn Feline Foundation Progress Report
http://www.winnfelinehealth.org/
By Susan Little DVM, Diplomate ABVP (Feline)

2001

Ultrasound Guided Percutaneous Ethanol Injection for the Treatment of Bilateral Hyperthyroidism in Cats
Investigators: E.C. Feldman, A.L. Wells, C.D. Long, W.J. Hornof
School of Veterinary Medicine,
University of California, Davis

Funded 1999

Hyperthyroidism is the most common endocrine disease of cats. About 95% of affected cats are over 10 years of age. It is caused by a benign tumor of the thyroid gland that triggers excess production of thyroid hormone. The thyroid gland is composed of two lobes and is located just below the larynx in the neck. In most cases, both lobes of the gland are affected (bilateral disease), but occasionally only one lobe is involved (unilateral disease).

Cats with hyperthyroidism may have a variety of clinical signs, including weight loss, increased appetite, increased thirst, vomiting and diarrhea. Congestive heart failure and high blood pressure may also be associated with this disease. Diagnosis is by palpation of the enlarged thyroid gland in the neck and by blood testing, including a thyroid hormone level.

There are three common treatment modalities for hyperthyroidism in cats. These are:

1. anti-thyroid medications, i.e. methimazole (Tapazole^®, Lilly)
2. radioactive iodine (I-131)
3. surgical removal of the gland (thyroidectomy)

Each of these methods is effective, but has its advantages and disadvantages. In human medicine, injection of small masses affecting the thyroid and parathyroid glands with ethanol has been used successfully. Percutaneous ethanol injection (PEI) has also been used to treat hyperparathyroidism in dogs.

Recently, researchers have been evaluating the use of PEI for the treatment of hyperthyroidism in cats. A study published in 1998 detailed PEI treatment of a cat and the associated adverse effects, such as laryngeal paralysis. In 1999, researchers presented information on using ultrasound guidance for PEI treatment of four hyperthyroid cats where only one lobe of the gland was affected. This approach proved to be safe and effective.

The purpose of this study was to determine if PEI treatment would be safe and effective for cats with bilateral hyperthyroidism. It was hoped that PEI would have advantages over the traditional treatments for hyperthyroidism, in that it would provide a permanent cure, would not involve daily administration of pills, would be less costly, and would involve less risk than surgery and less hospitalization than radioactive iodine.

The investigators used ultrasound to measure the size of the affected thyroid gland lobes and then calculated the amount of ethanol that would be injected into each lobe. Each cat was anesthetized, the hair over the thyroid gland was clipped, and the skin was prepared in a sterile manner as for surgery. Using ultrasound for guidance, the ethanol was injected into each lobe using a small gauge needle. The area was observed carefully via ultrasound for any leakage of the ethanol outside the gland.

The first patient treated was found inexplicably dead 12 hours after the procedure. Although a necropsy failed to find the cause of death, the researchers were concerned that it may have been due to bilateral laryngeal paralysis, which can lead to suffocation and death.

Subsequently, the procedures for the next 6 hyperthyroid cats were done in two stages. The largest thyroid lobe was done first and the second lobe was done no sooner than 30 days later. All cats were monitored carefully for several days after the procedures. Thyroid hormone levels as well as other blood parameters, such as blood calcium, were checked frequently. Ultrasound was also used to evaluate the size of the thyroid gland before and after the PEI procedure.

While all cats experienced a drop in thyroid hormone levels after PEI, the effect was only transient. Repeated treatments were performed in the cats, but the longest period any cat remained free of hyperthyroidism was 27 weeks. In the end, most cats required an alternate treatment for hyperthyroidism, such as surgery or anti-thyroid medication. In addition to the death of the first patient, transient mild adverse effects were seen in the remaining six cats that included Horner's syndrome, unilateral laryngeal paralysis and voice changes.

Unfortunately, the results of this study showed that PEI treatment for bilateral hyperthyroidism in cats cannot be recommended. The treatment proved to be less effective and less safe than currently available methods.

For further reading:

1. Goldstein, R., C. Long, et al. (1999). Ultrasound guided percutaneous ethanol injection (PEI) for the treatment of 4 cats with unilateral hyperthyroidism (abstract). Proc 17^th American College Veterinary Internal Medicine Forum, Chicago, IL.
2. Goldstein, R., C. Long, et al. (2001). Percutaneous ethanol injection for treatment of unilateral hyperplastic thyroid nodules in cats. J Amer Vet Med Assoc 218(8): 1298-1302.
3. Walker, M. and M. Schaer (1998). Percutaneous ethanol treatment of hyperthyroidism in a cat. Fel Pract 26(5): 10-12.
4. Wells, A., C. Long, et al. (1999). Ultrasound guided percutaneous ethanol injection (PEI) for the treatment of 6 cats with bilateral hyperthyroidism (abstract). Proc 17^th American College Veterinary Internal Medicine Forum, Chicago, IL.
5. Wells, A., C. Long, et al. (2001). Use of percutaneous ethanol injection for treatment of bilateral hyperplastic thyroid nodules in cats. J Amer Vet Med Assoc 218(8): 1293-1297.