Neonatal thyrotropin as measured in a congenital hypothyroidism screening program: influence of the mode of delivery.

Context: Many developed countries are reexperiencing iodine deficiency. One World Health Organization index of iodine deficiency in populations is the percentage of neonates with TSH levels greater than 5 mIU/liter 72 h after delivery. Measured TSH levels vary with methodology and are influenced by external factors including iodine exposure at time of delivery.
Objective: We wished to determine whether babies delivered by cesarean section have higher levels of TSH than babies delivered vaginally because this factor could influence determination of iodine deficiency.
Design and Setting: This was a cohort study of mothers delivering at a teaching hospital in 2002-2003 and their babies.
Patients and Methods: Women delivering a live infant were eligible for the study. Demographic data, mode of delivery, and subsequent routine newborn screening TSH levels were recorded.
Results: Of 2031 infants, 31.2% were delivered by cesarean section; 1864 babies were born after 37 wk, with blood samples collected by d 5 (>91% on d 3). Significant predictors of TSH concentration were the day TSH was measured and type of delivery. The distribution curve of TSH values was right shifted in infants born by cesarean section. The percentage of neonates born by cesarean with at TSH value greater than 5 was 7.1%. The figure for those born vaginally was 4.3%.
Conclusions: Babies delivered by cesarean section are significantly more likely to have TSH levels greater than 5 mIU/liter on d 3 than those delivered vaginally. With the rise in the rate of births by cesarean, this could be an important factor in assessing population iodine deficiency using neonatal TSH levels.