Thyroid function, maternal

G. N. Burrow, D. A. Fisher, and R R. Larsen Mechanisms of disease maternal and fetal thyroid function. New England Journal of Medicine 331, 1072 (1994). 

If a positive screening test is obtained, the mother s thyroid function is usually also assessed. Maternal autoantibodies can cross the placenta and block receptor sites on the fetal thyroid. In this rare situation, after an initial transient hypothyroidism just after birth, the baby s own thyroid function will usually develop normally. 

The fetus is totally dependent on maternal iodine supply throughout gestation, and on thyroxine supply during the first trimester of pregnancy for normal neurological development and nervous system maturation. It is therefore imperative that TH synthesis is adequate and is met with the appropriate iodine intake. Accordingly, it is important to know the trimester-specific reference intervals for THs and other thyroid functions in pregnancy. 

The observations relating to iodine deficiency in pregnancy are firstly those concerned with maternal thyroid function and maternal goiter. Maternal urinary iodine (UI) excretion is the usual method of assessing iodine status in the population at risk or the individual, and is discussed below. Neonatal indicators of maternal iodine deficiency are goiter and neurointellectual impairment. 

Recently, a WHO technical consultation group met to review the maternal iodine requirements during gestation (Benoist et al, 2007). Consideration was given to the fact that the amount recommended should be sufficient to prevent brain damage or thyroid function disorders due to iodine deficiency during pregnancy. There is a lack of data to indicate an optimal intake. Therefore, conclusions were reached after evaluation of the efficiency of gut absorption of iodine, the estimated metabolic needs, and the typical daily losses in the feces and urine. 

Gestational hypothyroidism has been associated with severe complications, such as hypertension, preterm birth, low birth weight, placental abruption and fetal death by Leung et al, (1993) and Allan et al. (2000). Many studies in children of hypothyroid pregnant women including Mans group (1976, 1991), and many later ones (Liu et al, 1994 Smit et al, 2000 Klein et al, 2001 Mitchell and Klein, 2004), have demonstrated the importance of maternal thyroid function on the neurodevelopmental evolution of the child. 

Our aim was to examine the neurodevelopment of a group of 3-year-old children of mothers living in an iodine-deficient area, and to establish the possible existence of a relationship between their performance and maternal iodine nutrition and thyroid function through gestation. 

Table 63.2 Maternal urine iodine excretion, thyroid function and TPO-Ab determination at the first and third trimester of gestation...

At 12 weeks of gestation, the UIE of 30 women (49%) was below 200 ig/l and 9 (15%) were below 100 j,g/l. Thirty-seven (60%) had iodized salt in thett diet. Eighty one percent of them had a UIE of > 200 pg/1, versus 33% of the women who did not have iodized salt p = 0.05). Table 63.2 shows maternal UIE, thyroid function and TPO-Ab at the first and third gestational trimester. 

The figure shows that a normal General Cognitive Index requires adequate maternal iodine nutrition conditions and a thyroid function. CGI, General Cognitive Index. UIE, urine iodine excretion. TSH, serum thyrotropin. 

Endemic cretinism has been a common finding in the mountains or isolated regions, such as the Alps or the Himalayas and has been described or depicted in drawings over the centuries. McCarrison (1917) studied this condition in the Himalayas, describing two types of cretinism the nervous and myxedematous types. He already suspected the influence of the maternal thyroid function on the fetus. Early observations in the Alps (Hunziker-Shild, 1915) already reported that the fetus could receive the hormone from the mother, and proposed the prophylactic administration of iodine from conception. 

Development of the neocortex can be affected by short periods of maternal hypothyroxinemia, before the onset of fetal thyroid function. Therefore, to maintain normal thyroxinemia in the mother is of maximal importance to maintain normal T3 levels in the fetal brain. 

The link between iodine-deficiency related minor neurological disorders and maternal thyroid function during gestation... 

Iodine deficiency and ADHD maternal thyroid function vs. ADHD... 

For the purposes of our study, the neuro-intellectual and behavioral outcome of all the ID and IS children was related to the thyroid function of their mothers over the first half of gestation. Average values of maternal T3, T4, free triiodothyronine (FT3), free thyroxine (FT4),TSH and TBG saturation by T4 at 8, 13 and 20 weeks of gestation in both the ID area ADHD+ and ADHD— subgroups,... 

When neurological results were related to early-pregnancy maternal thyroid function, it was seen that all 11 ADHD+ children were born to the ID area mothers whose thyroid function proved more heavily compromised than that of the 5 ADHD— mothers, and that 7/8 (87.5%) of the ID area mothers who experienced thyroid failure generated ADHD+ children. It is worth noting, however, that individual TSH levels fell consistently within the normal range in all but two of these women,... 

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