Seasonal Variation in Neonatal Thyroid Function

Abstract

In 226 full term infants serum T4, T3 test and serum TSH were estimated on their second or third day of life. The study was divided in two winter and two summer periods. The mean serum T4 values in the two summer periods were significantly lower than the mean values in the two winter periods (p < 0.01). Free T4 index values were also calculated and there was found a high significant difference with highest values in the winter (p < 0.001). No seasonal variation could be demonstrated in serum TSH on the second or third day of life. In a group of 20 full term infants, serum TSH was estimated at 30 min, 3 hr and 24 hr after birth, and it was demonstrated that serum TSH values 30 min after birth were significantly higher in two winter periods than in the summer (p < 0.05). Our data indicate that free T4 index values in the first days of life are higher in the winter than in the summer, and it seems possible that this difference is caused by the higher values of serum TSH found immediately after birth in children born in the winter period.