Potassium iodate and propranolol
In another recent study, researchers evaluated the use of a potassium iodate and propranolol combination in cats with hyperthyroidism
before surgical treat ment.6 Potassium iodate appears to affect thyroid metabolism by blocking the uptake of iodine and leading, ultimately, to a drop
in total T4 synthesis. Eleven cats (Group A) received propranolol on Days 1 through 10, followed by propranolol and potassium iodate
on Days 11 through 20. These cats initially received 2.5 mg propranolol given orally every eight hours. If their heart rates
exceeded 200 beats/min after four days, the dose was increased to 5 mg every eight hours. If the rates still exceeded 200
beats/min on Day 7, the dose was increased to 7.5 mg every eight hours. Ten cats (Group B) received only potassium iodate
(42.5 mg orally t.i.d.) on Days 1 through 10, and then propranolol was added using the same treatment regimen as Group A.
Surgery was performed on Day 21, and propranolol was continued for three days after surgery. The side effects of potassium
iodate included intermittent anorexia, vomiting, and mild depression in some cats (i.e. seven cats in Group A; six cats in Group B). When this occurred, the dose was reduced to 21.25 mg every eight hours. Blood
samples were obtained daily to measure concentrations of serum total T4, total T3, and reverse T3. The clinical signs improved in all cats during treatment. At surgery, 36% of the cats in Group A had normal serum total
T4 concentrations, while 89% with initially elevated total T3 concentrations had normal concentrations. In Group B, 10% of the cats had normal serum total T4 concentrations, while 75% with initially elevated total T3 concentrations had normal concentrations. The drug regimen used in Group A was better-tolerated and more effective in normalizing
thyroid hormone concentrations.
This potassium iodate and propranolol combination offers a medical alternative for those cats that cannot tolerate carbimazole
or methimazole. All the cats in the study underwent thyroidectomy, so long-term medical control with this protocol was not
studied. In addition, the use of iopanoic acid with propranolol should also be investigated because iopanoic acid appears
to have fewer gastrointestinal and hepatic side effects than potassium iodate.
Radioiodine therapy appears to be the safest and most effective treatment for hyperthyroidism. Radioiodine can be administered
intravenously, subcutaneously, or orally. While all routes appear effective, oral administration is discouraged because of
the risks of exposure to people administering the medication and environmental contamination. The availability of radioiodine
treatment has ex panded greatly in the past few years (Table 2). The cost of radioiodine treatment may initially seem prohibitive to pet owners, but advise them that long-term medical
treatment or surgery will result in similar treatment costs.
Selected Radioiodine Facilities in the United States
The largest study of the safety and efficacy of radioiodine treatment involved 524 cats.7 A scoring system based on three factors (severity of clinical signs, size of the thyroid gland, and serum T4 concentration) was used to select the dose of radioiodine to be administered subcutaneously: 310 cats (59%) received a low
dose of radioiodine (2.5 to 3.4 mCi; median = 3 mCi), 158 cats (30%) received a moderate dose (3.5 to 4.4 mCi; median =4 mCi),
and 56 cats (11%) received a high dose (4.5 to 5.4 mCi; median = 5 mCi). Serum T4 concentrations were still increased in 80 cats (15.3%) when they were discharged from the hospital, but by six months after
radioiodine administration, the serum T4 concentrations had dropped to within or below the reference range in all but eight cats (1.5%). Many cats had low serum T4 concentrations at some point after treatment, but only 11 cats (2.1%) developed clinical hypothyroidism and required L-thyroxine
supplementation. Thirteen cats (2.5%) had a relapse of hyperthyroidism 1.1 to 6.5 years after the initial radioiodine treatment.
The treatment response was good in 94.2% of the cats. Median survival time was two years; the percentage of cats alive after
one, two, and three years of treatment was 89%, 72%, and 52%, respectively.
Selected Radioiodine Facilities in the United States (cont'd)