Effects of methimazole on thyroid gland uptake of 99mTc pertechnetate in 19 hyperthyroid cats

Nineteen cats with abnormally high serum T-4 concentrations underwent thyroid scintigraphy using technetium-99m pertechnetate ((TcO4)-Tc-99m) before and after 36 +/- 6 days of methimazole administration (approximately 2.5 mg PO q 12 h). Thyroid-to-salivary gland ratios (T:S ratios) and percentage thyroidal uptake of injected radioactivity at 20 and 60 min after injection of (TcO4)-Tc-99m were compared before and after methimazole treatment. Serum thyroid stimulating hormone (TSH) concentration was measured before and after methimazole treatment. Quantitatively, there was a positive association between the thyroid uptake of (TcO4)-Tc-99m and the serum T-4 before treatment (r = 0.74-0.83). TSH suppression was present when cats were first evaluated for hyperthyroidism. Methimazole treatment did not relieve TSH suppression in 17 cats. Two cats with unilateral thyroid uptake developed bilateral, asymmetric thyroid uptake of (TcO4)-Tc-99m after treatment and had the greatest increase in TSH concentration after treatment. Quantitatively, thyroid scintigraphy did not significantly change after methimazole treatment (P > 0.1). Evaluation of serum TSH concentration may be helpful in identifying methimazole-induced changes in the scintigraphic features of hyperthyroidism in mildly hyperthyroid cats.