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

Abstract

Objectives Although radioiodine ( 131 I) is the treatment of choice for feline hyperthyroidism, 131 I-dosing protocols commonly induce iatrogenic hypothyroidism and expose azotaemia. A recently reported patient-specific 131 I dosing algorithm minimised the risk of 131 I-induced hypothyroidism and azotaemia, while maintaining high cure rates. The aim of the study was to report results of 131 I 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 131 I 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 99m Tc-pertechnetate (TcTU) were determined using thyroid scintigraphy. An initial 131 I 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 131 I uptake was measured, and additional 131 I 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 131 I 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 99m Tc-pertechnetate uptake ( P = 0.0002), whereas persistently hyperthyroid cats had higher severity scores ( P = 0.012). Conclusions and relevance Our results confirm that 131 I dosing with this new algorithm results in high cure rates, with a lowered prevalence of 131 I-induced overt hypothyroidism and azotaemia. Age, serum TSH concentrations, bilateral, symmetrical uptake and severity score help predict outcome.