Opposite Variations in Maternal and Neonatal Thyroid Function Induced by Iodine Supplementation during Pregnancy

Abstract

Whereas the consequences of extremes in iodine intake are well described, much less is known about the effect of more moderate variations in maternal iodine intake on fetal thyroid function. The present study performed in Denmark with mild to moderate iodine deficiency dealt with the effect of maternal iodine supplementation on thyroid function in the mother at term and in the fetus/neonate. Serum was collected consecutively from pregnant women at term (n = 144) and from cord blood (n = 139). Forty-nine women had a regular intake of vitamin and mineral tablets with iodine (150 microg/day) during pregnancy, and 95 took no artificial iodine supplementation. Iodine supplementation (+I) induced opposite variations in thyroid function in the mother and the fetus. In +I mothers, TSH was 7.6% lower than in mothers with no supplementation (P < 0.05). In cord blood, on the contrary, TSH was 27.3% higher in the +I group (P < 0.05). The variations were caused by opposite shifts in TSH frequency distribution in mothers and neonates. The association between iodine supplementation and high serum TSH in the neonates was further substantiated by an inverse correlation between thyroglobulin and TSH in cord blood (P < 0.001), whereas no specific pattern was observed in the mothers. High serum thyroglobulin was a marker of low iodine intake in both mothers and neonates. The results suggest that the fetal thyroid, at least in areas of mild iodine deficiency, is more sensitive to the inhibitory effect of iodine than hitherto anticipated.

Iodine supplementation during pregnancy of hypothyroid women treated with L-thyroxine neither influences neonatal TSH nor prevents decrease in maternal free thyroid hormone concentrations in second and third trimesters

“…However, in the group of hypothyroid women treated with L-thyroxine we did not show a positive influence of iodine supplementation on the foetal thyroid function parameter, which is the neonate TSH concentration. Similar results were obtained by other authors, who measured TSH concentrations in umbilical cord blood samples of children of healthy women receiving iodine supplementation in a dose of 150 μg daily [29]. Velasco et al also found an increase of neonatal TSH in a group of pregnant women receiving 50-200 μg of iodine daily, but based on psychomotor tests performed at 3-18 months of age he also showed that iodine supplementation positively influenced on the neurological development of children [30].…”

Thyroglobulin in smoking mothers and their newborns at delivery suggests autoregulation of placental iodide transport overcoming thiocyanate inhibition

“…This is a cross-sectional study carried out from November 1988 to March 1990 in five different cities in Denmark (14,24,30,31). As the time of study enrollment was before the mandatory Danish iodine fortification of salt introduced in the year 2000 (32), the population had in general mild (East Denmark) to moderate (West Denmark) iodine deficiency with the majority of the women under study living in an area of moderate iodine deficiency (78.6%).…”

The impact of iodised salt or iodine supplements on iodine status during pregnancy, lactation and infancy

“…Treatment had no significant effect on the concentration of maternal TSH, T 3 , T 4 , Tg or on the thyroid volume, and had no effect on the concentration of TSH in the newborn 51 . In a placebo-controlled, double-blind trial, Nohr et al 52 gave to 66 pregnant women who had antibodies to antithyroid peroxidase (TPO-Ab) a multi-nutrient supplement containing 150 mg iodine per day or a placebo control from the eleventh week of pregnancy to full term. The median UI concentration was significantly higher in the treated women at full term than in the controls, but there were no differences in the concentration of maternal TSH, fT 4 or Tg between groups 52 .…”

“…In a placebo-controlled, double-blind trial, Nohr et al 52 gave to 66 pregnant women who had antibodies to antithyroid peroxidase (TPO-Ab) a multi-nutrient supplement containing 150 mg iodine per day or a placebo control from the eleventh week of pregnancy to full term. The median UI concentration was significantly higher in the treated women at full term than in the controls, but there were no differences in the concentration of maternal TSH, fT 4 or Tg between groups 52 . There was no difference in the prevalence of anti-thyroglobulin antibodies (Tg-Ab) or TPO-Ab, and no differences between groups in the prevalence or severity of post-partum thyroid dysfunction (PPTD), defined as an abnormal TSH concentration in the ).…”