Muscular weakness in young patients is usually due to mild, self-limiting causes. Nonetheless, it is important to remember other, more serious aetiologies which can cause this clinical picture. Thyrotoxic hypokalaemic periodic paralysis (THPP) is a rare disease in Europe and the USA, with fatal cardiovascular and respiratory complications. It is characterised by recurrent episodes of generalised muscular weakness, especially in the legs, with an associated hypokalaemia and hyperthyroidism. Diagnosis is based on clinical history, laboratory tests and an ECG. Early treatment focused on cautious correction blood potassium and non-cardiac selective β-blockers. Additionally, it is imperative to normalise thyroid function to prevent relapses. We present a young, healthy man to the emergency department with episodes of intermittent leg weakness. The history and the ECG findings allowed for the diagnosis of THPP to be reached with early treatment causing remission.