TK2 and hyperinsulinemic hypoglycemia, familial, 4: Since the original description of TK2 deficiency, 63 additional patients have been described in isolated case reports and small series with heterogeneous clinical and molecular characteristics indicating that TK2 deficiency encompasses a disease spectrum.2–25 The lack of a comprehensive study depicting the natural history and defining the clinical, molecular and biochemical features has limited early recognition of the disease, prognostic information for patients and families, and planning for potential future clinical trials.