Relationship Between Maternal Iodine Status and Cord Blood Thyroid Function Profile in an Iodine Sufficient Environment

Abstract

Background: During the 10th to 12th week of gestation, foetal TSH appears and the foetal thyroid gland is capable of concentrating iodine and synthesizing iodothyronines. However, little hormone synthesis occurs until the 18th to 20th week. Thereafter, foetal thyroid secretion increases gradually. Foetal iodine is obtained from maternal sources and the extent to which maternal thyroid hormones cross the placenta is controversial. Aim: To determine the relationship between maternal iodine status with cord blood thyroid function test. Method: A prospective longitudinal study of two hundred and thirteen healthy pregnant women was conducted consisting of 58, 87 and 68 in the first, second and third trimesters respectively. Cord blood thyroid function tests (TSH, free T3 and free T4) were assayed using commercial ELISA kits while urinary iodine concentration (UIC) was assayed using a modified Sandell – Kolthoff reaction. Median UIC of less than 150, 150 – 249 and above 249 µg/L were considered inadequate, adequate and more than adequate respectively. Data was analysed using Epi – info 3.5.3 and p – value less than 0.05 was considered significant. Results: Cord blood TSH mean (SD) was significantly elevated 9.44(7.4) µIU/L in cord blood of mothers with inadequate iodine status compared with those having adequate 7.21(8.4) µIU/L and more than adequate 6.81 (5.5) µIU/L (p = 0.0029). There was a significantly positive correlation between maternal UIC and cord blood TSH (P = 0.0000, r = 0.26) but not with free T4. Cord blood TSH suggestive of congenital hypothyroidism (> 20 µIU/L) was seen in 9 out of 90 (10 %) of those with insufficient, 2/31 (6.5 %) of those with sufficient and 3/92 (3.3 %) of those with more than adequate iodine statuses. Conclusion: Cord blood TSH relates positively with maternal iodine status.