Thyroid storm, also known as thyrotoxic crisis, is an uncommon and potentially life-threatening condition that can cause metabolic, cardiovascular and neuropsychiatric complications. Thyrotoxic crisis often develops in the context of untreated or poorly controlled hyperthyroidism and is typically precipitated by acute stressors such as infection, surgery, or trauma. While the progression from hypothyroidism to hyperthyroidism is itself an uncommon clinical occurrence, the emergence of thyroid storm in this setting is sporadic. Such a case highlights the importance of being aware of atypical presentations of thyroid storm. The association between thyrotoxicosis and Moyamoya disease has been reported. There is no published case report or review describing Moyamoya syndrome in association with the transformation of hypothyroidism to hyperthyroidism.

A 28-year-old patient with a known history of hypothyroidism, type 2 diabetes mellitus (T2DM) on SGLT2 inhibitors, polycystic ovarian syndrome (PCOS) and mixed anxiety-depressive disorder presented to the accident and emergency department with palpitations, chest discomfort and focal seizures reported by her partner and family. Initial blood tests showed euglycemic diabetic ketoacidosis (eDKA) and a very high thyroid function test suggestive of thyrotoxicosis. A CT of the brain showed a developing right frontal lobe infarct. The patient's condition continued to deteriorate despite ICU input. A repeat brain CT suggested Moyamoya syndrome. The patient died despite early detection and thorough, multidisciplinary care input from endocrinology, neurology, stroke and critical care teams.

This example underpins the poor prognosis of Moyamoya syndrome with time and again coexists with mental illness such as anxiety and depression, even with early identification and management, the need for multispecialty cooperation, and the difficulties in treating the uncommon neuropsychiatric complication of thyrotoxicosis.

The case highlights how acute psychiatric and behavioural change may be the first signal of catastrophic neurological disease in thyrotoxicosis. It underlines the importance of recognising neuropsychiatric deterioration as a medical emergency and supports early neurovascular assessment and integrated neuroendocrine and mental health care in complex presentations.

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