Prior to therapy, patients choosing I131 therapy for the treatment of Graves disease were pre treated with carbimazole. Post I131 they have their thyroid function checked monthly in primary care prior to clinic review at 3 months. Carbimazole can be restarted if clinically indicated and treatment with thyroxine was recommended if their TSH was greater than 3.5 mU/l (ref range 0.355.5). A previous audit had demonstrated that 24% of patients had an elevated TSH greater than 5.5 mU/l on more than one occasion prior to commenting thyroxine. In 2016 a number of the endocrinologists started using block and replace (B&R) prior to I131. B&R was discontinued 7 days prior to treatment and recommenced 7 days after treatment. Carbimazole was discontinued at 6 months and thyroxine continued lifelong if indicated. We reviewed the last 2 years outcome data to determine if there was a difference in post treatment hypothyroidism (defined as TSH>5.5 mU/l at 6 months) or relapse rate. (Defined as treatment with carbimazole at 12 months post therapy). Since 2016, fifty seven patients have been treated for Gravess disease of which 22 used B&R and 35 standard carbimazole pre-treatment. TSH >5.5 mU/l at 6 months in the carbimazole and B&R groups were 34% and 18% respectively and relapse rate at 12 months 17% and 9%. Thus, B&R post I131 therapy resulted in a similar cure rate and was less likely to result in an elevated TSH at 6 months.