A dosing algorithm for individualized radioiodine treatment of cats with hyperthyroidism. (5th August 2021)

• Record Type: Journal Article
• Authors: Peterson, Mark E.; Rishniw, Mark
• Is Part Of: Journal of veterinary internal medicine. Volume 35:Number 5(2021)
• Page Start: 2140
• Page End: 2151
• Publication Date: 2021-08-05
• Subjects: 131I -- feline -- hypothyroidism -- radioactive iodine -- scintigraphy -- thyroid gland -- thyroid‐stimulating hormone -- thyroxine -- triiodothyronine; Veterinary medicine -- Periodicals; 636.0896
• DOI: 10.1111/jvim.16228
• Languages: English
• ISSNs: 0891-6640

Abstract

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 (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 99m Tc‐pertechnatate (TcTU) were determined. An initial 131 I 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 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 T4, 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.