We report two patients with type 1 diabetes mellitus, previously well contr
olled with good compliance, presenting with unexplained diabetic ketoacidos
is. Following initial correction of the metabolic disorder, persisting tach
ycardia lead to the diagnosis of thyrotoxicosis. In both cases, treatment w
ith propranolol and carbimazole helped in the stabilization of their metabo
lic states. Although thyrotoxicosis is known to destabilise diabetes contro
l, we can find no reports of it precipitating diabetic ketoacidosis.