A dosing algorithm for individualized radioiodine treatment of cats with hyperthyroidism

Abstract Background Radioiodine (131I) is the treatment of choice for hyperthyroidism in cats, but current 131I‐dosing protocols can induce iatrogenic hypothyroidism and expose azotemia. Objectives To develop a cat‐specific algorithm to calculate the lowest 131I dose to resolve hyperthyroidism, while minimizing risk of iatrogenic hypothyroidism and subsequent azotemia. Animals One thousand and four hundred hyperthyroid cats treated with 131I. Methods Prospective case series (before‐and‐after study). All cats had serum concentrations of thyroxine (T4), triiodothyronine (T3), and thyroid‐stimulating hormone (TSH) measured (off methimazole ≥1 week). Using thyroid scintigraphy, each cat's thyroid volume and percent uptake of 99mTc‐pertechnatate (TcTU) were determined. An initial 131I dose was calculated by averaging dose scores for T4/T3 concentrations, thyroid volume, and TcTU; 80% of that composite dose was administered. Twenty‐four hours later, percent 131I uptake was measured, and additional 131I administered, as needed, to deliver an adequate radiation dose to the thyroid tumor(s). Serum concentrations of T4, TSH, and creatinine were determined 6 to 12 months later. Results The median calculated 131I dose was 1.9 mCi (range, 1.0‐10.6 mCi); 1380 cats required additional 131I administration on day 2. Of the cats, 1047 (74.8%) became euthyroid, 57 (4.1%) became overtly hypothyroid, 240 (17.1%) became subclinically hypothyroid, and 56 (4%) remained hyperthyroid. More overtly (71.9%) and subclinically (39.6%) hypothyroid cats developed azotemia than euthyroid cats (14.2%; P < .0001). Conclusions and Clinical Importance Our algorithm for calculating individual 131I doses resulted in cure rates similar to historical treatment rates, despite much lower 131I doses. This algorithm appears to lower prevalence of both 131I‐induced overt hypothyroidism and azotemia.