This is the first case report of upper extremity lymphoedema caused by a thyroid tumour. An 87 year old female patient with grade 3 lymphoedema of the upper right extremity was admitted. The lymphoedema had developed over the course of two years. Investigation identified a thyroid tumour, which compressed the mediastinal structures at the right superior thoracic outlet, causing venous congestion, oedema, and lymphoedema. The patient underwent thyroidectomy which markedly improved the lymphatic oedema. Resolution of the compression mechanism was an effective treatment, despite the severity and chronicity of the initial presentation.