IGSF1 and Macrocephaly: Although all evaluated patients had CCeH, which may also cause macrocephaly, evaluation of head circumference in a larger group of IGSF1-deficient patients including available data from the previously published cohort (total n = 41, ages 0.4-89.5 years) confirmed that this did not correlate with free thyroxine levels at the time of evaluation (P = 0.8) or with age at commencement of levothyroxine (P = 0.4).