Graves disease (GD) is the most common cause of hyperthyroidism1 and is caused by stimulating autoantibodies to the TSH receptor (TRAb).2 TRAb assays have 98% sensitivity and 99% specificity 3 for GD and are recommended as the first-line cost-effective investigation to diagnosis the aetiology of hyperthyroidism and determine the risk of thyroid eye disease.4,5 In TRAb antibody negative patients, radionuclide imaging may be helpful in demonstrating focal increased uptake (toxic nodular disease) or absent uptake (thyroiditis). We audited our diagnostic work-up of new cases of hyperthyroidism presenting to Kings College Hospital, London between 1 January 2018 to 30 June 2018. We identified 56 patients (48 female and 8 male); age range 2092 years. TRAbs were measured in n=44 patients: TRAbs were elevated in n=21, measurable but within the reference range in n=8, and undetectable in n=15. Of the n=15 TRAb negative patients; n=6 had convincing clinical/biochemical evidence of a thyroiditis; n=3 demonstrated spontaneous remission of thyrotoxicosis and n=2 underwent technetium uptake scans which showed features typical of GD. N=4 patients did not attend requested imaging investigations. Of the n=8 patients with measurable but normal range TRAbs, n=2 were managed as GD, n=3 underwent technetium uptake scans which showed features typical of GD, n=2 had clinical/biochemical features suggestive of a thyroiditis and n=1 demonstrated resolution of thyrotoxicosis. N=2 patients underwent nuclear medicine imaging as a first-line diagnostic investigation to establish the aetiology of hyperthyroidism; n=1 revealed an autonomously functioning nodule and n=1 nodules with no focal autonomy. N=10 patients had neither TRAb nor nuclear medicine imaging as first line investigation; of these n=5 had evolving/sub-clinical disease, n=5 were observed. In conclusion, TRAb testing was performed first-line in 79% of new referrals for hyperthyroidism. We are working towards clinicians adopting a more consistent approach to determining the aetiology of hyperthyroidism.