A 14-yr-old female presented with diabetes and Graves' disease. Eighteen months later, she was euthyroid on carbimazole, and her haemoglobin A1c (HbA1c) was normal (5.2%) on a small insulin dose (0.3-0.4 units/kg/day). An assessment of her pancreatic beta-cell reserve, determined by comparing HbA1c and insulin dose, suggested that this was greater than other patients with type 1 diabetes in our service 18 months postdiagnosis (n = 185). We suspect that excess thyroid hormone led to an insulin-resistant state and accelerated her presentation with hyperglycaemia. Insulin resistance fell once normal thyroid function was restored and helped to attenuate further beta-cell destruction when beta-cell mass was relatively well preserved.