Hypothyroidism affects about 2% of all women but occurs in only about 0.5% of pregnant women. The discrepancy is probably due to the known association between hypothyroidism and infertility. Other causes of inadequate thyroid function during and after pregnancy include iodine deficiency, Hashimoto’s disease, thyroidectomy, radioactive iodine treatment, and subacute. Inadequate treatment of hypothyroidism can have serious consequences for both the mother and fetus. Hypothyroidism during pregnancy has been associated with pregnancy-induced hypertension, placental abruption, postpartum hemorrhage, and an increase in the frequency of low birth weight infants.
A study published in 1999 examined the association of hypothyroidism in mothers and neurocognitive development in their children. Serum concentrations of thyroid stimulating hormone (TSH) were measured in 25,216 pregnant women and 62 had a TSH result that was greater than 98th percentile, suggesting that they had clinical or subclinical hypothyroidism. These 62 women were then matched with 124 healthy women and 15 tests of IQ were determined in their 7-9 year old children. The children from the 62 women with thyroid disease performed slightly less well than the control children on all 15 IQ tests. 48 of the 62 women with thyroid disease were not treated for their hypothyroidism and the children from those women had significantly lower IQ scores than the control children.
The study suggests an association between an underactive thyroid gland during pregnancy and delayed neurodevelopment in the offspring and begs the question:
"Should all pregnant women be screened for hypothyroidism?"
Several medical associations have weighed in on this subject. Guidelines from the American Association of Clinical Endocrinologists, indicate that TSH screening should be routine before pregnancy or during the first trimester. If the TSH is greater than 10 mU/L or if the TSH is 5-10 mU/L and the patient has goiter or positive anti-thyroid peroxidase antibodies, then thyroid hormone replacement therapy should be initiated.
The American Thyroid Association and the Endocrine Society agree that there are not enough data for or against universal screening but also acknowledge that just because there is no evidence of benefit doesn’t mean that there is no benefit. They recommend the screening of pregnant women who are at high risk of overt hypothyroidism (e.g. history of thyroid dysfunction, TPO antibody positive, goiter etc). If the TSH is greater than 10 mU/L, this indicates overt hypothyroidism, and thyroid hormone replacement therapy should be initiated.
However, the American Congress of Obstetricians and Gynecologists has recommended against screening all pregnant women for hypothyroidism. They argue that there is lack of clear evidence that the identification and treatment of women with subclinical hypothyroidism will improve maternal or infant outcomes.
To date, there is no clear evidence to suggest that the treatment of pregnant women with subclinical hypothyroidism prevents neurodevelopmental in their offspring. Perhaps a clinical trial funded by the National Institute of Child Health & Human Development will clear away the controversy.