The primary objective of this clinical trail is to evaluate whether early supplementation of levothyroxine (L-T4) at a dosage of 8 ug/kg/d, initiated in the second week of life, improves physical growth and neurodevelopmental outcomes at 2 years of corrected age in Small for Gestational Age (SGA) infants with birth weights < 1500g and TSH levels between 6 and 20 mIU/ml. Preterm infants in the intervention group will receive 8 ug/kg/d of L-T4, while the control group will receive no supplementation. Physical growth will be assessed through height, weight, and head circumference, while neurodevelopmental status will be evaluated using the Bayley-IV scales and cranial MRI imaging findings.

Inclusion Criteria:

• Preterm infants born Small for Gestational Age (SGA) with a birth weight < 1500g；
• Admitted to a participating Neonatal Intensive Care Unit (NICU) within 7 days of birth；
• Serum concentrations of TSH, FT4, and FT3 are measured weekly during the first and second weeks of life，any single TSH measurement during this period is between 6 and 20 mIU/mL；
• Written informed consent obtained from the parents.

Exclusion Criteria:

• Diagnosis of Congenital Hypothyroidism (CH) with serum TSH > 20 mIU/mL within the first 2 weeks of life and receiving L-T4 treatment;
• Persistent serum TSH < 6 mIU/mL throughout the first 2 weeks;
• Elevated thyroid hormone levels(T3, T4, FT3, or FT4) above the reference range within the first 2 weeks;
• Diagnosis of Grade III or IV Intraventricular Hemorrhage (IVH) within the first 2 weeks of life;
• Presence of major congenital malformations of vital organs or known chromosomal/genetic disorders;
• History of maternal thyroid disease or maternal use of thyroid-related medications.