A 67-year-old Caucasian woman with no prior medical history was admitted to our hospital with complaints of generalised weakness, nausea, diarrhoea and weight loss. The patient suffered from tachycardia and hypotension. Blood tests revealed Graves' thyrotoxicosis and the patient was treated accordingly. However, patient's health continued to decline rapidly and further tests revealed a concomitant Addisonian crisis. Additional treatment with corticosteroids led to a full recovery. It is well known that autoimmune endocrine disorders tend to cluster. However, the presentation is usually sequential in time. This case reports the highly rare simultaneous presentation of Addison's disease and Graves' thyrotoxicosis. It also provides several suggestions to help establish the diagnoses.