Thyroid function in healthy thai neonates

Objectives: To construct a normative data for serum thyroxine (T₄), free T₄ (FT₄), triiodothyronine (T₃) and thyrotropin (TSH) in Thai neonates. Study design: A cross-sectional study of 275 healthy full-term neonates was conducted. Blood samples were obtained from umbilical cords of the neonates and from heel pads of infants aged 1-30 days. Hormone measurements included serum T₄, FT₄, T₃ and TSH. Results: Mean serum T₄ and FT₄ levels rapidly increased after delivery to the maximum level at 1-3 days of age. Thereafter, they declined to a steady state level within 2-4 weeks. Mean serum T₃ level was very low at birth. The concentration increased 3-5 times and reached a steady state levels within 1 week. In contrast, mean serum TSH declined from birth and the level at 1-3 days of age was slightly less than that of the cord blood. It changed little after 3 days of age. Previous studies have shown a transient TSH surge in the first 24-48 hour of life. TSH surge was not apparent in our study because samples were not obtained from infants <24 hours old. Therefore, if TSH is measured for screening of congenital hypothyroidism, samples should be obtained from umbilical cord or infants aged >48 hours. Conclusions: This study provides the normative data for thyroid function tests in Thai full-term neonates. These data are useful for detection and verification of hypothyroidism in a screening program for congenital hypothyroidism.