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

BACKGROUND

Radioiodine ( I) is the treatment of choice for hyperthyroidism in cats, but current I-dosing protocols can induce iatrogenic hypothyroidism and expose azotemia.

OBJECTIVES

To develop a cat-specific algorithm to calculate the lowest I dose to resolve hyperthyroidism, while minimizing risk of iatrogenic hypothyroidism and subsequent azotemia.

ANIMALS

One thousand and four hundred hyperthyroid cats treated with I.

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 ≥1 week). Using thyroid scintigraphy, each cat's thyroid volume and percent uptake of Tc-pertechnatate (TcTU) were determined. An initial I dose was calculated by averaging dose scores for T /T concentrations, thyroid volume, and TcTU; 80% of that composite dose was administered. Twenty-four hours later, percent I uptake was measured, and additional I 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 months later.

RESULTS

The median calculated I dose was 1.9 mCi (range, 1.0-10.6 mCi); 1380 cats required additional 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 I doses resulted in cure rates similar to historical treatment rates, despite much lower I doses. This algorithm appears to lower prevalence of both I-induced overt hypothyroidism and azotemia.