Abstract
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.