Improving obstetric outcome: antenatal thyroid screening

Mild hypothyroidism, where plasma levels of thyroid stimulating hormone (TSH) are above the ‘normal’ upper limit but where there is no equivalent change in circulating levels of the thyroid hormones tetraiodothyronine (T4) and triiodothyronine (T3), is common in women of childbearing age; the condition is found in up to 3?% of pregnant women. While normally asymptomatic, in pregnant women mild hypothyroidism has been associated with miscarriage, perinatal death and preterm delivery, the major cause of neonatal death. Several studies have investigated whether treatment with levothyroxine, a synthetic thyroid hormone, would improve the obstetric outcome in women with borderline thyroid function, and results from the most recent study were reported at the Society for Endocrinology (BES) conference in November.
In this study, 645 women out of more than 13?000 tested at the end of the first trimester of pregnancy were found to have sub-clinical hypothyroidism (340) or isolated hypothyroxinemia (305). In the latter condition TSH levels are normal but T4 levels are below the lower reference limit. Five hundred and eighteen women with abnormal thyroid function took part in a randomized trial, with half being prescribed levothyroxine and half acting as control. Rates of stillbirth, neonatal death and delivery before 34 weeks were compared, as well as delivery between 34 and 37 weeks and cesarean sections carried out before 37 weeks. It was found that untreated women with abnormal thyroid function had an increased risk of stillbirth, delivery before 37 weeks and having an early cesarean section when compared with women with normal thyroid function and those treated with the synthetic thyroid hormone. Although the authors emphasize that larger trials are needed to confirm their findings, it seems likely that this cheap and safe drug could have a significant impact on obstetric outcome.
In the more developed countries thyroid autoimmunity is the main cause of hypothyroidism, with iodine deficiency being less frequent. Thyroid autoantibodies, particularly thyroid peroxidase antibodies (TPO), can be measurable even in women with biochemically normal thyroid function, and are a risk factor for miscarriage and preterm delivery. Elevated levels are found in up to 20?% of women, but also in as many as 31?% of sub-fertile women. There is a dearth of robust studies to assess the effect of levothyroxine on pregnancy outcomes in these women but it could be that measuring TPO in both sub-fertile as well as pregnant women, followed by treatment with levothyroxine if indicated, could result in many more healthy, full-term babies.