Methods: We retrospectively measured triiodothyronine (T3), thyroxine (T4), and reverse T3 (rT3) levels in DBS samples obtained at 4–5 days of life from 6 infants with genetically confirmed MCT8 deficiency and from 110 controls. The gene discussed is SLC16A2; the disease is hyperinsulinemic hypoglycemia, familial, 4.