Introduction: Graves’ disease (GD) and toxic multinodular goitre (TMNG) are the most common causes of thyrotoxicosis presenting to an endocrinology service. In New Zealand (NZ) the usual definitive therapy is radioactive iodine (I¹³¹). Not all patients are cured by a single dose of I¹³¹. Features reported to decrease the effectiveness of I¹³¹ include male gender, young age and a large gland (1-4). Our clinical impression is that Maori patients are also more resistant to I¹³¹ treatment. This has not previously been reported.

Method: Retrospective review of I¹³¹ for thyrotoxicosis at Waikato Hospital, NZ during the 3 year period prior to 1 December 2010.

Results: A total of 326 doses of I¹³¹ were given to 285 patients. Follow up data were available on 283 patients. Median follow-up was 858.5 days (range 30-1525). 83.4% were female. Mean age was 53.12 years (± 14.96) years. Maori comprised 32% of the I¹³¹ group and Caucasians 55%. The diagnosis was GD for 61.1% and TMNG for 34.3%. Most patients (98.2%) received a fixed dose of 555mBq I¹³¹. At last follow up cure had been achieved in 72.1%. There was no difference in cure rates according to gender (p=0.1212) or diagnosis (GD 70.5% vs TMNG 73.2% p=0.6377). Younger patients (<50 years) were less likely to achieve cure as compared to >50years (63.7% vs 78.6% respectively, p=0.0059). Maori patients were less likely to achieve cure than Caucasian patients (60.4% vs 77.1% respectively, p=0.0058). Maori were also more likely to be younger (49.80 ± 11.29 years vs 55.84 ± 16.04, p=0.0018) and more likely to have a TMNG (47.3% vs 31.2% p=0.0124).

Discussion: A fixed dose of I¹³¹ successfully cured hyperthyroidism in 72% of patients treated. In the population studied, age <50 years and Maori ethnicity predicted a lower rate of cure from a single dose of I¹³¹ therapy.