A 32-year-old woman presents with palpitations, tremor, and heat intolerance for 3 weeks. Examination reveals a diffuse goitre, lid lag, and thyroid bruit. TSH is suppressed at 0.01 mIU/L (normal 0.4–4.0), free T4 is elevated at 28 pmol/L (normal 10–20), and free T3 is 8.5 pmol/L (normal 3.5–6.5). Anti-TPO and anti-thyroglobulin antibodies are negative. Technetium-99m thyroid uptake is uniformly increased. What is the most appropriate next step in management?