A 20-year-old lady of Vietnamese origin presented with increasing frequency of headaches and worsening right sided weakness over 4 days. There was no other significant past medical illness, and she was not on any regular medication. She was not a smoker, and there was a history of thyrotoxicosis in her mother. Her heart rate was 110 beats per minute, and she was in sinus rhythm. On examination there was right sided weakness (MRC grade 4/5), expressive dysphasia, tremor of her extended fingers and smooth thyromegaly. CT angiogram showed smooth moderate narrowing throughout the course of the left internal carotid artery (ICA) and narrowing of the intracranial right ICA, right MCA and ACA. MRI showed ischaemic changes involving the left ICA territory. She was started on dual anti-platelets. Subsequent investigations revealed the following: FT3 41.7 pmol/l(3.1-6.8), FT4 >100 pmol/l(11-21.2) and TSH suppressed, TSH receptor antibody: 16.1 U/l(0-1.75), anti-TPO ab: 110 IU/ml (0-35). She was commenced on Carbimazole 40 mg once daily and Propranolol 80 mg three times a day (which was later switched to Verapamil to address any potential cerebral vasospasm). Repeat CT angiogram done 5 days later, for worsening right sided weakness (MRC grade 0/5) showed worsening changes affecting the proximal major intracranial vessels with complete occlusion of the distal left ICA. An extensive vasculitis and thrombophilia screen did not reveal any abnormalities. An FDG PET-CT and MRA to assess extra-cranial vasculature did not show any evidence of large vessel vasculitis. Upper and lower limb arterial duplex study revealed triphasic waveforms throughout. CSF biochemistry and microbiology were unremarkable. There was no surgical target or indication for vascular bypass and medical treatment was advised. Differentials include Graves disease associated cerebral vasculopathy and Moyamoya syndrome, both of which are uncommon. She exhibited a limited responsive to Carbimazole and therefore switched to Propylthiouracil 200 mg four times a day. She received 3 doses of intravenous methylprednisolone 1000 mg and then was switched to oral prednisolone 50 mg titrating down by 5 mg weekly to a maintenance dose of 10 mg daily and was started on Methotrexate 15 mg subcutaneous weekly to be continued for 1 year. At the time of discharge, the weakness and expressive dysphasia had considerably improved. FT4 was 40.7 pmol/l, FT3 9.1 pmol/land TSH remained suppressed. Benefits of long-term immunosuppression are still unclear, with any treatment likely to prevent further stenosis rather than reverse changes already present.