TG and hyperthyroidism: Mother–infant dyad studies, which demonstrate both maternal hypothyroidism and hyperthyroidism resulting in small for gestational age infants, indicate the importance of an optimal balance of the thyroid hormone [44,45,47,52]; however, the optimal value for preterm infants has not yet been elucidated [13], and hormone intervention in preterm infants may be interfered with by the therapeutic guidelines for infants with congenital hypothyroidism.