A dosing algorithm for individualized radioiodine treatment of cats with hyperthyroidism.

Abstract

BACKGROUND: Radioiodine (I) is the treatment of choice for hyperthyroidism in cats, but currentI-dosing protocols can induce iatrogenic hypothyroidism and expose azotemia. OBJECTIVES: To develop a cat-specific algorithm to calculate the lowestI dose to resolve hyperthyroidism, while minimizing risk of iatrogenic hypothyroidism and subsequent azotemia. ANIMALS: One thousand and four hundred hyperthyroid cats treated withI. METHODS: Prospective case series (before-and-after study). All cats had serum concentrations of thyroxine (T), triiodothyronine (T), and thyroid-stimulating hormone (TSH) measured (off methimazole &#x2265;1&#xa0;week). Using thyroid scintigraphy, each cat's thyroid volume and percent uptake ofTc-pertechnatate (TcTU) were determined. An initialI dose was calculated by averaging dose scores for T/Tconcentrations, thyroid volume, and TcTU; 80% of that composite dose was administered. Twenty-four hours later, percentI uptake was measured, and additionalI administered, as needed, to deliver an adequate radiation dose to the thyroid tumor(s). Serum concentrations of T, TSH, and creatinine were determined 6 to 12&#x2009;months later. RESULTS: The median calculatedI dose was 1.9&#xa0;mCi (range, 1.0-10.6&#xa0;mCi); 1380 cats required additionalI 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&#x2009;<&#x2009;.0001). CONCLUSIONS AND CLINICAL IMPORTANCE: Our algorithm for calculating individualI doses resulted in cure rates similar to historical treatment rates, despite much lowerI doses. This algorithm appears to lower prevalence of bothI-induced overt hypothyroidism and azotemia.