The options for treating feline hyperthyroidism include surgery, medical therapy, and radioiodine therapy. The choice among these options is based on the animal's clinical status (the severity of clinical signs and the presence of concurrent nonthyroidal illness), the costs, the clinician's experience, and the availability of radioiodine therapy.
Surgical removal of enlarged, autonomously functioning thyroid glands is frequently used to treat hyperthyroidism, especially when radioiodine therapy is not an option or a cat experiences side effects from long-term oral or topical medication. Assess affected cats carefully before surgery to detect concurrent medical conditions, such as renal or cardiac disease and hypertension. Since both thyroid glands are affected in more than 80% of hyperthyroid cats, bilateral thyroidectomy is usually necessary. Several thyroidectomy techniques have been developed to minimize the postoperative complications of bilateral thyroidectomy, such as hypocalcemia or recurrence of hyperthyroidism.
Damage to or removal of all four parathyroid glands during bilateral thyroidectomy results in hypocalcemia, the most common postoperative complication.1 To counteract this complication, researchers have evaluated the efficacy of autotransplanting parathyroid tissue in normal cats.2 They performed bilateral thyroidectomy and parathyroidectomy with parathyroid autotransplantation in eight healthy, random-source adult cats.2 They found that serum calcium concentrations normalized faster postoperatively than concentrations reported in cats undergoing bilateral thyroidectomy and parathyroidectomy. Parathyroid autotransplantation greatly reduced morbidity in cats that had undergone a parathyroidectomy. Autotransplantation should be considered in cats in which all four parathyroid glands have been removed or in cases in which the surgeon is concerned that accurate identification of all parathyroid tissue was not possible during bilateral thyroidectomy. The researchers also found that transplanted normal thyroid tissue was present in at least three of the eight cats. This indicates that, when performing autotransplantation in hyperthyroid cats, it is important to remove all associated thyroid tissue to prevent the recurrence of hyperthyroidism.
Some clinicians advocate sequential removal of bilaterally affected thyroid glands. Staging a bilateral thyroidectomy presumably allows time for ipsilateral parathyroid tissue to revascularize before the second thyroid gland is removed and the blood supply to the contralateral parathyroid glands is interrupted. However, little information has been published to show the long-term efficacy and safety of staged thyroidectomies.
Medically treating hyperthyroidism involves the use of methimazole, carbimazole (a pro-drug to methimazole available outside of the United States), and iodine-containing agents such as ipodate, iopanoic acid, and iodate. These forms of medical therapy may be used alone or with b-blockers, such as propranolol hydrochloride.
Should you use transdermal methimazole in hyperthyroid cats?
Methimazole is currently the drug most commonly used to treat hyperthyroidism. It is effective as an oral drug but can also be administered transdermally (see boxed text titled "Should you use transdermal methimazole in hyperthyroid cats?"). Methimazole prevents iodine from being incorporated into the tyrosyl groups of thyroglobulin, the precursor of thyroid hormones. The medication may have side effects, including vomiting, self-excoriation, anorexia, hepatopathy, and hematologic abnormalities. The drug is given twice a day and is often used to normalize serum thyroid hormone concentrations before treating a cat with surgery or radioactive iodine. It appears that twice-a-day dosing is necessary because a recent report demonstrated that euthyroidism was restored in 87% of cats treated twice a day vs. 54% of cats treated once a day. No difference in the rate of gastrointestinal side effects or facial pruritus was noted between the two groups.3 See Table 1 for recommendations on diagnosing hyperthyroidism, laboratory testing, initiating methimazole therapy, and patient monitoring.
Initial Treatment of Hyperthyroid Cats with Methimazole
An Australian study confirmed the efficacy and safety of carbimazole in the medical treatment of feline hyperthyroidism.4 The study included 25 cats in which hyperthyroidism was diagnosed during a 23-month period. Carbimazole therapy was instituted with owner consent, and rechecks were scheduled two, six, and 13 weeks after beginning therapy. The cats underwent a physical examination and hematologic and serum chemistry testing at each reexamination. Owners were also asked to assess clinical signs in the periods between rechecks. Fourteen cats treated with carbimazole responded favorably to therapy during the 13-week monitoring period. Side effects were minor and uncommon. Cats with underlying renal disease that became apparent during the study responded well to alterations in or cessation of carbimazole therapy.