Context: Many developed countries are re-experiencing iodine deficiency. One WHO index of iodine deficiency in populations is the percentage of neonates with thyroid stimulating hormone (TSH) levels > 5 mIU/L 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 caesarean section have higher levels of TSH than babies delivered vaginally, as this factor could influence determination of iodine deficiency.

Design and setting: 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 caesarean section. 1864 babies were born after 37 weeks, with blood samples collected by day 5 (> 91% on day 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 caesarean section. The percentage of neonates born by caesarean, with at TSH value > 5 was 7.1%. The figure for those born vaginally 4.3%.

Conclusions: Babies delivered by caesarean section are significantly more likely to have TSH levels greater than 5 mIU/L on day 3 than those delivered vaginally. With the rise in the rate of births by caesarean, this could be an important factor in assessing population iodine deficiency using neonatal TSH levels.