IgG4-related disease of the thyroid gland is a recently recognised subtype of thyroiditis, often with rapid progression requiring surgical treatment. Here, we report a 47-year-old Malay woman presenting with progressively painless neck swelling over three years, with subclinical hypothyroidism. CT scan revealed diffuse thyroid enlargement (up to 13 cm) with retrosternal extension and without regional lymphadenopathy. Fine needle aspiration of the thyroid showed limited well-visualised follicular epithelial cells with widespread Hurthle cell change and scanty background colloid, but no evidence of lymphocytosis. The cytologic features fell into the category of ‘atypia of undetermined significance’. Subsequently, the patient developed hypercapnic respiratory failure secondary to extrinsic upper airway compression by the thyroid mass and underwent emergent total thyroidectomy. Histology of the thyroid showed diffuse dense lymphoplasmacytic infiltrate with fibrosis and focal obliterative phlebitis. Follicular cells exhibited reactive nuclear features and some Hurthle cell change. IgG4+ plasma cells were over 40/high power field while overall IgG4/IgG ratio was above 50%. Post-surgery, the patient was found to have markedly elevated serum IgG4 concentration but PET/CT did not show significant increased fludeoxyglucose uptake in other areas. Her recovery was complicated by a ventilator-associated pneumonia with empyema, limiting early use of corticosteroids on treatment of IgG4-related disease.