This is a review article of serological tests in thyroid eye disease, IgG4-related disease and myasthenia gravis. The authors provide an overview of the current serology with a brief description of how they relate to the pathogenesis of each condition. The relevance of the various thyroid hormones and autoantibodies in the diagnosis and management of Grave’s orbitopathy are explained, with an emphasis on the role of TSH receptor antibodies, and in particular the difference between stimulating, binding and blocking antibodies. There is a summary of IgG4-related disease with a simple review of immunoglobulins and the diseases in which the various sub-classes of IgG may be elevated or deficient. Absolute levels of IgG4 in the blood are of limited value as the normal range can vary between individuals by a factor of 100 or more. However, the ratio of IgG4+ to IgG+ plasma cells may be a more sensitive diagnostic tool with values over 40% being significant. Tissue levels are also useful, with >10IgG4+ plasma cells per high power field being the cut-off (>100 in the lacrimal gland). Other conditions including granulomatosis with polyangiitis and sarcoidosis may also have elevated IgG4, and the authors stress interpreting the results within the context of the broad clinical picture. Up to 40% of patients with IgG4-related disease will have normal serum IgG4 levels, which may be explained by the prozone effect. This is an artefact in the test due to massive concentrations of immunoglobulin affecting the binding to the test antigen. There is a brief review of myasthenia gravis and its clinical subtypes, with reference to testing for acetylcholine receptor (AChR), muscle-specific kinase (MuSK) and lipoprotein receptor-related protein 4 (LRP4) antibodies. Purely ocular myasthenia occurs in 10-15%, and only 50% of these will have AChR or MuSK antibodies. Conversion from ocular to generalised myasthenia can occur up to two years after onset, and seroconversion may also be delayed so repeat testing is often indicated in suspected cases. This is a useful review of these autoimmune conditions. Part two, yet to be published, will cover sarcoidosis, vasculitis, Sjogren’s and giant cell arteritis.