TSH receptor antibody (TRAb) can be broadly divided into three functional categories: stimulating, neutral or blocking. It is generally recognised that stimulating TRAb is the pathogenic agent of thyrotoxicosis in Graves’ disease (GD). However, TSH receptor antibody assays widely adopted in Australian laboratories are TRAb binding immunoassays that do not distinguish the functional characteristics of the antibody. Cell based Thyroid Stimulating Immunoglobulin-cyclic AMP assay (TSI-cAMP) was the gold standard for measuring the functional activity of TRAb, but this was withdrawn from the market in 2011. In 2009, FDA approved a qualitative TSI bioassay, Thyretain®, which utilizes genetically engineered Chinese Hamster Ovary cells expressing a chimeric form of the human TSH receptor (hTSHR) and a cyclic AMP induced luciferase reporter gene to determine functional signalling. More recently, FDA approved another semi-quantitative chemiluminescent immunoassay, Immulite®2000 TSI Assay (I-2000TSI), which captures TSI using chimeric hTSHR and signals with a second alkaline phosphatase labelled chimeric receptor. Here we report the first head to head comparison of the manual BRAHMS TRAK® (TRAK), a popular competitive binding radioimmunoassay, versus I-2000TSI and Thyretain® assays. One hundred seventeen samples collected from the TSH Receptor Antibody in Thyroid Disease (TRAB) Study at our Hospital between 2013 and 2016 were analysed. Ninety-eight active GD, 7 thyroiditis, and 12 negative controls were included. All three assays show excellent sensitivity above 95% (Table 1). Thyretain® outperforms the other two assays at correctly excluding non GD conditions with positive TRAb samples, whereas the I-2000TSI and TRAK had specificity of 59% and 68% respectively (both P<0.0001 compared with Thyretain). Furthermore, Thyretain® was able to detect stimulating activity in 4 out of 5 TRAb negative and clinically definite GD samples. In summary, this study shows that Thyretain® bioassay appears to have particular utility in cases with low titre TRAb.