Radioiodine treatment of hyperthyroidism in cats: results of 165 cats treated by an individualised dosing algorithm in Spain

Abstract

Objectives Although radioiodine (131I) is the treatment of choice for feline hyperthyroidism, 131I-dosing protocols commonly induce iatrogenic hypothyroidism and expose azotaemia. A recently reported patient-specific 131I dosing algorithm minimised the risk of 131I-induced hypothyroidism and azotaemia, while maintaining high cure rates. The aim of the study was to report results of 131I treatment in a European population of hyperthyroid cats using this patient-specific dosing algorithm. Methods This prospective case series (before-and-after study) evaluated 165 hyperthyroid cats referred for 131I treatment. All cats had serum concentrations of thyroxine (T4), triiodothyronine (T3) and thyroid-stimulating hormone (TSH) measured (off methimazole ⩾1 week). Thyroid volume and percentage uptake of 99mTc-pertechnetate (TcTU) were determined using thyroid scintigraphy. An initial 131I dose was calculated by averaging dose scores for T4/T3 concentrations, thyroid volume and TcTU; 70% of that composite dose was then administered. Twenty-four hours later, percentage 131I uptake was measured, and additional 131I administered as needed to deliver an adequate radiation dose to the thyroid tumour(s). Serum concentrations of T4, TSH and creatinine were determined 6–12 months later. Results Median calculated 131I dose was 2.15 mCi (range 1.2–7.5), with only 51 (30.9%) receiving ⩾2.5 mCi. Of 165 cats, 124 (75.2%) became euthyroid, seven (4.2%) became overtly hypothyroid, 27 (16.4%) became subclinically hypothyroid and seven (4.2%) remained hyperthyroid. A higher proportion of overtly (85.7%) and subclinically (26.9%) hypothyroid cats developed azotaemia than euthyroid cats (13.6%; P = 0.0002). Hypothyroid cats were older ( P = 0.016) and more likely to have detectable TSH concentrations ( P = 0.025) and symmetrical bilateral distribution of 99mTc-pertechnetate uptake ( P = 0.0002), whereas persistently hyperthyroid cats had higher severity scores ( P = 0.012). Conclusions and relevance Our results confirm that 131I dosing with this new algorithm results in high cure rates, with a lowered prevalence of 131I-induced overt hypothyroidism and azotaemia. Age, serum TSH concentrations, bilateral, symmetrical uptake and severity score help predict outcome.

Radioiodine therapy with individualized dose protocol in a feline patient with hyperthyroidism

“…Ademais, o exame de cintilografia se mostra útil nos casos suspeitos, em que o valor de T4 total se encontra dentro do intervalo e referência em pacientes hipertireoideus, possibilitando o diagnóstico da doença (Peterson, 2013). O conjunto de informações oferecidos pela cintilografia possibilita a realização da radioterapia utilizando o protocolo de doses individualizadas proposto por Peterson & Rishniw (2021) e replicado por Xifra et al (2022). Ambos os trabalhos relataram taxa de eutireoidismo em felinos pós terapia de 74,8% e 75,2%, respectivamente.…”

Blood pressure in hyperthyroid cats before and after radioiodine treatment

“…Based on these results, we classified the cats as nonazotemic versus azotemic (serum creatinine >2.0 mg/dL). We also classified the cats as being euthyroid versus hypothyroid, based on the following criteria: euthyroid (T 4 , 1.0‐3.8 μg/dL; TSH ≤0.30 ng/mL), overtly hypothyroid (T 4 < 1.0 μg/dL; TSH >0.30 ng/mL), and subclinically hypothyroid (T 4 , 1.0‐2.5 μg/dL; TSH >0.30 ng/mL), as previously described 41,48‐50 . Cats that remained persistently hyperthyroid were excluded (Figure 1).…”

“…We also classified the cats as being euthyroid versus hypothyroid, based on the following criteria: euthyroid (T 4 , 1.0-3.8 μg/dL; TSH ≤0.30 ng/mL), overtly hypothyroid (T 4 < 1.0 μg/dL; TSH >0.30 ng/mL), and subclinically hypothyroid (T 4 , 1.0-2.5 μg/dL; TSH >0.30 ng/mL), as previously described. 41,[48][49][50] Cats that remained persistently hyperthyroid were excluded (Figure 1). All these 131 I-treated cats had SBP measured again, by the protocol described above.…”