A dosing algorithm for individualized radioiodine treatment of cats with hyperthyroidism

Abstract

Background Radioiodine (131I) is the treatment of choice for hyperthyroidism in cats, but current 131I‐dosing protocols can induce iatrogenic hypothyroidism and expose azotemia. Objectives To develop a cat‐specific algorithm to calculate the lowest 131I dose to resolve hyperthyroidism, while minimizing risk of iatrogenic hypothyroidism and subsequent azotemia. Animals One thousand and four hundred hyperthyroid cats treated with 131I. 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 99mTc‐pertechnatate (TcTU) were determined. An initial 131I 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 131I uptake was measured, and additional 131I 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 131I dose was 1.9 mCi (range, 1.0‐10.6 mCi); 1380 cats required additional 131I 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 131I doses resulted in cure rates similar to historical treatment rates, despite much lower 131I doses. This algorithm appears to lower prevalence of both 131I‐induced overt hypothyroidism and azotemia.

Predicting outcomes in hyperthyroid cats treated with radioiodine

“…We recently published the results of a large cohort of hyperthyroid cats treated with individualized 131 I doses using a novel algorithm, calculated to administer the lowest effective dose possible. 7 With this dosing algorithm, we achieved euthyroidism at rates similar to previous studies using conventional dosing protocols (>95%), despite much lower 131 I doses, without increasing the probability of treatment failure (persistent hyperthyroidism). With this approach, the prevalence of overt hypothyroidism fell below 5%, but nearly 20% of 131 I‐treated cats still developed mild (subclinical) hypothyroidism.…”

“…The clinical details of the 1400 hyperthyroid cats in our study have been previously reported. 7 To be eligible for inclusion, cats underwent an evaluation that included a complete physical examination, routine laboratory testing (CBC, serum biochemical profile, complete urinalysis), determination of serum thyroid hormone concentrations (total thyroxine [T 4 ], triiodothyronine [T 3 ], and thyroid‐stimulating hormone [TSH]), 2 , 8 , 9 and qualitative and quantitative thyroid scintigraphy. 10 , 11 In the 728 cats treated with methimazole, owners discontinued administration of the drug ≥1 week (median, 7 days; interquartile range [IQR], 7‐15 days) before evaluation and treatment with 131 I.…”

“… 10 , 11 In the 728 cats treated with methimazole, owners discontinued administration of the drug ≥1 week (median, 7 days; interquartile range [IQR], 7‐15 days) before evaluation and treatment with 131 I. 7 Owners feeding a low‐iodine diet (Hill's Prescription Diet y/d Feline, Topeka, Kansas) were instructed to feed an iodine‐replete diet for at least 4 weeks before treatment. We excluded cats with pre‐existing azotemia (defined as serum creatinine concentration > 2.0 mg/dL), as well as cats with multifocal thyroid disease (≥3 separate tumor nodules or areas of increased radionuclide uptake), a scintigraphic pattern that indicates a higher probability of thyroid carcinoma.…”

“…The full details of the 131 I dosing algorithm used to treat the 1400 cats in our study have been previously reported. 7 In brief, we calculated individual 131 I doses for all cats, based on pretreatment serum T 4 and T 3 concentrations, estimated thyroid volume (measured from scintigraphic image), and the percent uptake of 99m Tc‐pertechnetate ( 99m TcO 4 − ). These variables contributed to a severity score, from which we calculated the 131 I dose.…”

“…With this approach, the prevalence of overt hypothyroidism fell below 5%, but nearly 20% of 131 I‐treated cats still developed mild (subclinical) hypothyroidism. 7 Therefore, the outcome of 131 I treatment remains difficult to predict with certainty and is far from perfect. Identifying factors that predict undesired outcomes could allow clinicians to modify treatment protocols and improve treatment outcomes.…”