Prematurity is associated with altered central thyroid hormone sensitivity.

OBJECTIVES: Thyroid Feedback Quantile-based Index (TFQI), the Thyrotropin-Thyroxine Resistance Index (TT4RI), and the Thyrotropin Index (TSHI) have been developed to assess central sensitivity to thyroid hormones by integrating circulating thyroid hormone levels with the pituitary thyrotropin response. This study aimed to investigate central and peripheral thyroid hormone sensitivity in preterm neonates using established thyroid hormone sensitivity indices. METHODS: This study included 70 neonates monitored in our hospital's neonatal intensive care unit (NICU), categorized into two groups: those born at <32 weeks of gestation (n=17) and those born at ≥32 weeks (n=53). Clinical characteristics and thyroid function test results were recorded. Peripheral thyroid hormone sensitivity was assessed using the Free T3 (FT3)/Free T4 (FT4) ratio, while central thyroid hormone sensitivity was evaluated using TT4RI, TSHI, and TFQI, with lower FT3/FT4 and higher TT4RI, TSHI, and TFQI values indicating reduced thyroid hormone sensitivity. RESULTS: The mean postnatal age of neonates born at <32 weeks (n=17; 9 females, 52.9%) was 92.50 ± 21.34 days, compared with 58.09 ± 20.21 days for those born at ≥32 weeks (n=53; 26 females, 49.0%). TSHI, TT4RI, and TFQI, were significantly lower in neonates born at <32 weeks compared with those born at ≥32 weeks. CONCLUSIONS: Lower thyroid hormone sensitivity indices in early preterm neonates suggest altered central thyroid hormone regulation despite normal conventional thyroid function tests. These findings underscore the clinical importance of incorporating sensitivity indices into the evaluation of preterm neonates and suggest that further monitoring of their metabolic and cardiovascular status is warranted.