A dosing algorithm for individualized radioiodine treatment of cats with hyperthyroidism

BACKGROUND: Radioiodine ((131)I) is the treatment of choice for hyperthyroidism in cats, but current (131)I‐dosing protocols can induce iatrogenic hypothyroidism and expose azotemia. OBJECTIVES: To develop a cat‐specific algorithm to calculate the lowest (131)I dose to resolve hyperthyroidism, while minimizing risk of iatrogenic hypothyroidism and subsequent azotemia. ANIMALS: One thousand and four hundred hyperthyroid cats treated with (131)I. METHODS: Prospective case series (before‐and‐after study). All cats had serum concentrations of thyroxine (T(4)), triiodothyronine (T(3)), and thyroid‐stimulating hormone (TSH) measured (off methimazole ≥1 week). Using thyroid scintigraphy, each cat's thyroid volume and percent uptake of (99m)Tc‐pertechnatate (TcTU) were determined. An initial (131)I dose was calculated by averaging dose scores for T(4)/T(3) concentrations, thyroid volume, and TcTU; 80% of that composite dose was administered. Twenty‐four hours later, percent (131)I uptake was measured, and additional (131)I administered, as needed, to deliver an adequate radiation dose to the thyroid tumor(s). Serum concentrations of T(4), TSH, and creatinine were determined 6 to 12 months later. RESULTS: The median calculated (131)I dose was 1.9 mCi (range, 1.0‐10.6 mCi); 1380 cats required additional (131)I 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 (131)I doses resulted in cure rates similar to historical treatment rates, despite much lower (131)I doses. This algorithm appears to lower prevalence of both (131)I‐induced overt hypothyroidism and azotemia.