Since remission of hyperthyroidism does not ever occur in cats, the aim of treatment is to normalize the excessive secretion of thyroid hormone by the cat's thyroid gland. Because all thyroid cats have a thyroid tumor (98% are benign but are few are malignant), a secondary aim of treatment is to remove or destroy the cat's thyroid tumor(s).
Hyperthyroidism can be treated medically, surgically, or with radioiodine. Medical treatment consists of administering antithyroid drugs (methimazole) one to three times per day. Methimazole treatment usually controls the signs of hyperthyroidism, but it is not a cure (i.e., the drug will block thyroid hormone secretion but will not remove or destroy the thyroid tumor). The drug must be given for the rest of the cat's life. Surgery cures the hyperthyroid condition by removing part or all of the thyroid gland. Radioiodine (radioactive iodine; I-131) also cures hyperthyroidism. The procedure for this treatment is simple; it is a single subcutaneous injection of a radioiodine, given much like a routine vaccine.
The advantages and disadvantages of these 3 treatment options are outlined below.
Medical Treatment with Anti-Thyroid Drugs (Methimazole; Tapazole™; Felimazole™)
Advantages
• Relatively inexpensive (in the short-term) compared with radiotherapy or surgery.
• No hospitalization required.
• No permanent hypothyroidism.
• No hypoparathyroidism or hypocalcemia (low calcium crisis secondary to damage of parathyroid
glands)
Disadvantages
• The price of trade name methimazole has risen dramatically in the last few years, making the total
cost of long-term administration equivalent to that of definitive treatments.
• Medication must be given daily (usually twice a day). Pilling your cat several times daily can
damage your relationship with your cat.
• Relapses are common during long-term treatment, and some cats respond poorly to the medication.
• Approximately 25% of cats experience side effects, especially lethargy, loss of appetite, and
vomiting.
• Serious complications are rare but include facial itching, liver failure, low white blood cell count,
and anemia.
• Continual monitoring and blood tests at 3- to 6-month intervals are necessary during long-term
treatment. This adds to the expense of treating with methimazole.
• The disease is not cured; the thyroid tumor continues to grow in size and may transform from a
benign tumor to a cancerous tumor over time.
Surgical Treatment (Thyroidectomy)
Advantages
• Usually successful; persistent hyperthyroidism rare.
• Corrects hyperthyroid state in shortest time (1-2 days).
• Cure of hyperthyroidism usually permanent.
Disadvantages
• Surgery is moderately expensive.
• Hospitalization is required (1-3 days).
• Performing general anesthesia and surgery has inherent risks. Furthermore, these risks are increased
in cats with heart disease, which is common in both hyperthyroid and geriatric cats.
• This is the most technically difficult treatment option. It is often difficult to identify and completely
remove all involved thyroid tissue, especially when the tumor is located in an ectopic location
(outside the neck area) such as the chest cavity.
• The parathyroid glands are little glands that lie adjacent to the thyroid that control the body’s
calcium balance. During thyroid surgery, they may be damaged or accidentally removed, resulting
in hypocalcemia (low calcium crisis).
• Other serious complications may include vocal cord paralysis and Horner's syndrome caused by
injury to the sympathetic nerve trunk in the neck.
• If all thyroid tumor tissue is not removed, hyperthyroidism is likely to persist or recur shortly after
surgery.
Radioiodine (Radioactive Iodine; I-131)
Advantages
• Highest rate of cure for hyperthyroidism
• Destroys thyroid tumors regardless of location
• Cure of hyperthyroidism usually permanent; lowest rate of recurrence
• Simplest treatment; single injection, usually given subcutaneously (under the skin)
• No daily pilling
• No anesthesia
• No serious complications
• Limited monitoring required after successful treatment; no ongoing therapy
Disadvantages
• Radioiodine is moderately expensive.
• Hospitalization is required (5 days to 2 weeks).
• Owner cannot visit their cat during the hospitalization.
• Most facilities require that the cat confined indoors and have limited contact with owners for a
period of time after discharge.
Radioiodine...The Best Solution for Feline Hyperthyroidism?
Radioiodine treatment completely cures hyperthyroidism by destroying the thyroid tumor and nothing else. Over 95% of all cats treated do not need future treatment. The procedure for this treatment is simple; it is a single subcutaneous injection of a radioiodine, given much like a routine vaccine.
Radioiodine therapy has some distinct advantages over medical or surgical treatment, and virtually all authorities consider it to be the treatment of choice for hyperthyroidism. With radioiodine, there is no need for anesthesia and the risk of postoperative hypocalcemia is eliminated. Methimazole (Tapazole™ or Felimazole™) treatment is not needed. The major drawback is that after receiving radioiodine, the cat must be kept in the hospital for a period (usually 5 to 7 days). Overall, radioiodine provides a simple, effective, and safe cure for cats with hyperthyroidism, regardless of age.
Dr. Peterson was the first veterinarian to develop this procedure for use in hyperthyroid cats. He has over 30 years of experience successfully administering radioiodine therapy to cats. It is not a new or experimental treatment; Dr. Peterson has treated over 10,000 hyperthyroid cats in his long career.
