Thyroid Function Test in Preterm Neonates: Normative Data.

Introduction: Initial surge of thyroid-stimulating hormone (TSH) in neonates increases free and total triiodothyronine (T3) and tetraiodothyronine (T4) in 24-36 hours following birth, and the effect then gradually wanes off. As somatic and intellectual development is dependent on normal thyroid function especially in infancy, normative data in these children may be of immense value to diagnose hypothyroidism in this subset of infants. Comprehensive normative values of thyroid function parameters in preterm neonates are scarcely available. The objective of this study was to determine the normative value of thyroid function parameters in preterm neonates.

Methods: Preterm neonates (n = 102) born at 34 and 35 weeks of gestation of euthyroid mothers from an iodine-sufficient population were evaluated for T3, T4, free thyroxine (FT4) and TSH during 3-7 days after birth and again after 1 month. The expected date of delivery (EDD) and Ballard score were used to identify the duration of gestation.

Results: The mean gestational age was 34.7 ± 0.41 weeks. The mean (± SD) for T3 (ng/dl), T4 (μg/dl), FT4 (ng/ml) and TSH (μIU/ml) on days 3-7 following birth was as follows: 156 ± 44.6, 12.8 ± 3.7, 1.50 ± 0.54 and 7.13 ± 6.04, respectively. Around 4 weeks of age, values changed to 104 ± 38.4, 12.1 ± 4.02, 1.46 ± 0.42 and 3.25 ± 2.85, respectively. All parameters changed significantly around 4 weeks, except FT4. None of the parameters were correlated with gestational age or body weight at birth. Normative values for each parameter in percentiles were generated.

Conclusion: This study generated the normative values of the thyroid function test during the first week and after around 4 weeks of life for premature neonates (born at 34-35 weeks).