Methimazole may produce side effects in cats including depression, vomiting and lack of appetite. These signs usually resolve without stopping the medication. A more serious side effect is the development of low blood cell counts which are more likely to develop during the first 3 months of treatment. Blood cell counts should be evaluated every 2 weeks during the first 3 months. If blood cell counts decrease methimazole is stopped and another treatment method should be considered.
Cats can have ectopic thyroid tissue in locations other than the neck.
Enlarged thyroid glands can be surgically removed. Methimazole is given for 1 to 2 months before surgery so that thyroid hormone levels are normal at the time of surgery. A few cats will become hypothyroid and may need to take thyroid pills. Occasionally complications may develop including damage to the parathyroid glands, which are closely attached to the thyroid gland, damage to nerves close to the thyroid gland or damage to the voice box. Parathyroid gland damage causes low blood calcium that may cause seizures.