SECISBP2 and hyperinsulinemic hypoglycemia, familial, 4: In severe cases of these rare diseases, measurement of T4 and T3 would allow for the detection of elevated T4 in SECISBP2 deficiency and elevated T3 in MCT8 deficiency, but in many cases, especially in THRα deficiency and mild cases of MCT8 and SECISBP2 deficiency, T4 and T3 levels are within the normal range and the diagnosis may be missed.