Objective:To investigate the reference ranges for thyroid function and its influencing factors in preterm infants at 14 d after birth.Methods:This retrospective study involved 514 preterm infants who met the inclusion criteria in Affiliated Hospital of Inner Mongolia Medical University from January 1, 2019 to December 31, 2021. They were divided into three group according to their gestational age [early premature group (26-31 +6 weeks, n=153), middle premature group (32-33 +6 weeks, n=129) and late premature group (34-36 +6 weeks, n=232)] or birth weight (BW) [<1 500 g group ( n=129), 1 500-2 000 g group ( n=120) and ≥2 000 g group ( n=265)]. Venous blood samples were collected from the infants at 14 d after birth and their thyroid function was determined by chemiluminescence immunoassay. The reference values of free triiodothyronine (FT 3), free thyroxine (FT 4) and thyroid stimulating hormone (TSH) were calculated based on the values of 95% confidence intervals ( CI) and expressed as percentiles in the range from P2.5 to P97.5. Mann-Whitney U test or Kruskal-Wallis H test was used to compare those thyroid hormone levels between groups. Spearman correlation analysis was used to study the correlation of gestational age or birth weight with FT 3, FT 4 and TSH levels. The factors influencing the levels of thyroid hormones were analyzed by multiple linear regression. Results:The reference ranges for FT 3, FT 4 and TSH were 1.53-3.72 pg/ml, 0.81-1.91 ng/dl and 1.32-7.80 μIU/ml in the early premature infants, 1.74-4.16 pg/ml, 0.90-2.82 ng/dl and 0.63-7.64 μIU/ml in middle prematures and 2.07-4.88 pg/ml, 1.09-2.27 ng/dl and 1.14-7.06 μIU/ml in late prematures. The reference ranges for the above three indexes were 1.53-4.06 pg/ml, 0.81-1.83 ng/dl and 1.14-7.84 μIU/ml in premature infants with BW<1 500 g, 1.67-3.98 pg/ml, 0.88-2.97 ng/dl and 0.94-7.64 μIU/ml in those whose BW between 1 500 g and 2 000 g and 1.91-4.75 pg/ml, 1.09-2.31 ng/dl and 1.14-6.32 μIU/ml in those whose BW≥2 000 g. Multiple linear regression showed that the level of FT 3 was positively correlated with gestational age ( β=0.119, P<0.05) and birth weight ( β=1.950×10 －4, P<0.05); that of FT 4 was positively correlated with gestational age only ( β=0.031, P<0.05); and TSH level was negatively correlated with birth weight ( β=－4.250×10 －4, P<0.05). Conclusions:Gestational age and birth weight are the factors influencing thyroid function in preterm infants at 14 d after birth. Evaluation of thyroid function with FT 4 and TSH should based on the references ranges of different gestational age and birth weight .