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

Abstract

OBJECTIVES: Although radioiodine (I) is the treatment of choice for feline hyperthyroidism,I-dosing protocols commonly induce iatrogenic hypothyroidism and expose azotaemia. A recently reported patient-specificI dosing algorithm minimised the risk ofI-induced hypothyroidism and azotaemia, while maintaining high cure rates. The aim of the study was to report results ofI 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 forI 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 ofTc-pertechnetate (TcTU) were determined using thyroid scintigraphy. An initialI 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, percentageI uptake was measured, and additionalI 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 calculatedI 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%; = 0.0002). Hypothyroid cats were older ( = 0.016) and more likely to have detectable TSH concentrations ( = 0.025) and symmetrical bilateral distribution ofTc-pertechnetate uptake ( = 0.0002), whereas persistently hyperthyroid cats had higher severity scores ( = 0.012). CONCLUSIONS AND RELEVANCE: Our results confirm thatI dosing with this new algorithm results in high cure rates, with a lowered prevalence ofI-induced overt hypothyroidism and azotaemia. Age, serum TSH concentrations, bilateral, symmetrical uptake and severity score help predict outcome.