Iodine supply and thyroid function in women with gestational diabetes mellitus: a cohort study

Maternal urinary iodine concentration and blood neonatal thyroid-stimulating hormone (TSH) concentration reflect iodine status in pregnancy and serve as markers of iodine deficiency. As dietary measures in gestational diabetes mellitus (GDM) could affect iodine intake, our study aimed to investigate iodine supply in women with GDM compared to healthy pregnant women and to evaluate its relationship to maternal and neonatal thyroid function.

Urinary iodine concentration (UIC) and serum TSH, free thyroxine (FT4), and autoantibodies against thyroid peroxidase (TPOAb) were analyzed in 195 women with GDM and 88 healthy pregnant women in the second trimester. Subsequently, neonatal TSH concentrations measured 72 h after delivery in a subgroup of 154 newborns (115 of mothers with GDM and 39 controls) from the national register were analyzed.

Median UIC was significantly lower in women with GDM compared to controls (89.50 µg/L vs. 150.05 µg/L; P < 0.001). Optimal iodine intake was found only in nine women with GDM (4.6%) and 33 healthy pregnant women (37.5%) (P < 0.001). Most pregnant women with GDM (88.7%) compared to one half of controls (50%) had iodine deficiency (P < 0.001). Although serum TSH and the prevalence of hypothyroidism (TSH > 4.0 mIU/L) were not different in both groups, hypothyroxinaemia was more prevalent in GDM compared to controls (12.3% vs 3.4%, P = 0.032). Consistently, neonatal TSH > 5.0 mIU/L indicating iodine deficiency, was found in 6 (5.2%) newborns of women with GDM as compared to none in controls. In women with GDM, the prevalence of perinatal complications was significantly lower in those who were taking dietary iodine supplements compared to those who were not (3/39 (7.69%) vs 46/156 (28.85%), P <0.001). In the multiple logistic and linear regression models in women with GDM, hypothyroxinaemia was associated with preterm births, and a negative association of serum FT4 and HbA1c was found.

Iodine deficiency in pregnancy was more prevalent among women with GDM compared to healthy pregnant controls. Serum FT4 negatively correlated with HbA1c, and hypothyroxinaemia was associated with preterm births in women with GDM. Conversely, women with GDM who used dietary iodine supplements had a lower risk of perinatal complications.