Postpartum Maternal Hyperthyrotropinemia in an Area in Which Iodine Supplementation is Required

Abstract

Mild maternal hypothyroidism during pregnancy can adversely affect infant development. We studied thyrotropin (TSH) levels in mothers of premature and low-birth-weight infants in Colombia, where iodized salt supplements the diet to correct iodine deficiency. The additional impact of salt restriction in mothers with hypertensive disorders was examined. Blood was spotted on filter paper from 404 mothers and their infants. Using radioimmunoassay (RIA), TSH was measured in the mothers, and TSH and thyroxine in their infants at three postpartum times. Initially, mothers had high TSH levels (i.e., TSH > 10 mU/L in half the mothers at the first assessment). Fourteen days later, only 9.3%, and at calculated term 7.5% were greater than 10 mU/L. Maternal TSH levels correlated with infant birth weight and gestational age (r = 0.47, and r = 0.49, p < 0.01). Initial TSH values were higher in salt restricted (20.1 ± 2 mU/L, n = 76) versus control mothers (14.6 ± 0.85, n= 328, p < 0.01), dropping dramatically in both groups 14 days later (to 3.4 ± 0.7 mU/L vs. 2.8 ± 0.4 mU/L) and at calculated term (2.8 ± 0.4 mU/L vs. 2.3 ± 0.6 mU/L). Increased maternal TSH levels during pregnancy in an iodine-deficient area may be aggravated by salt restriction. Monitoring TSH and supplementing iodine or thyroxine are recommended in pregnancy, especially if dietary salt restriction is prescribed.