Autoimmune Polyglandular Syndrome Type II Presenting with Severe Hyponatraemia and Autoimmune Bicytopaenia.

Background: Autoimmune polyglandular syndrome type II (APS II) is a rare, multifactorial disorder characterised by a combination of primary adrenal insufficiency with other autoimmune endocrinopathies, most commonly thyroid disease and type 1 diabetes mellitus. The clinical presentation is variable, and non-specific symptoms often lead to delayed diagnosis. Case description: We report the case of a 37-year-old female patient with a history of Graves-Basedow thyroid disease status post total thyroidectomy, admitted for dyspeptic symptoms, vomiting and severe hyponatraemia (112 mmol/l). Endocrinological evaluation revealed hypercortisolism with elevated adrenocorticotropic hormone (ACTH), reduced insulin, C-peptide and dehydroepiandrosterone sulfate (DHEAS) levels, and adrenal atrophy on MRI. Serological tests demonstrated IgA deficiency and positive anti-dsDNA antibodies of the IgM class. Additionally, the patient presented with bicytopaenia associated with autoimmune neutropaenia and detectable anti-granulocyte and anti-lymphocyte antibodies. The patient was diagnosed with APS II and started on hydrocortisone and fludrocortisone replacement therapy, resulting in subsequent clinical stabilisation. Conclusion: In this case, Addison's disease represented the first clinical manifestation of APS II and presented with severe hyponatraemia. A rare aspect of this case was the presence of bicytopaenia with autoimmune neutropaenia, which is not a typical feature of APS II and thus expands its phenotypic spectrum. This case highlights the importance of early recognition of endocrine causes of severe hyponatraemia and demonstrates that APS II can also manifest with rare haematologic complications. Long-term follow-up is essential due to the risk of progression or the development of additional autoimmune disorders. LEARNING POINTS: Severe hypotonic hyponatraemia can be the dominant presenting feature of primary adrenal insufficiency; therefore, adrenal insufficiency should be actively considered in the differential diagnosis of profound hyponatraemia, especially when accompanied by gastrointestinal symptoms and volume depletion.Autoimmune polyglandular syndrome type II may rarely be associated with autoimmune cytopaenias (including neutropaenia); detection of anti-granulocyte/anti-lymphocyte antibodies and selective IgA deficiency should prompt haematologic and immunologic follow-up for evolving or additional autoimmune manifestations.