Adequate thyroid hormone availability is important for an uncomplicated pregnancy and optimal fetal growth and development. Thyroid disease in pregnancy is a common clinical problem. Overt thyroid disease is associated with a wide range of adverse obstetric and developmental problems in the child. An increasing number of studies indicate that even milder form of thyroid dysfunction may have poor obstetric outcomes.
Subclinical hypothyroidism (SCH), a mild form of hypothyroidism defined as elevated TSH with normal free thyroxine levels, is a common diagnosis among women of reproductive age. In some, but not all, studies, it has been associated with infertility, an increased risk of adverse pregnancy and neonatal outcomes, and possibly with an increased risk of neurocognitive deficits in offspring. Despite well-established recommendations on treatment of overt hypothyroid pregnant women, a consensus has not yet been reached on whether to treat women with SCH. This presentation focuses on examining the evidence informing the clinical strategy for using levothyroxine (LT4) in women with SCH during pregnancy and those who are planning conception. A crucial first step is to accurately diagnose SCH using the appropriate population-based reference range. For pregnant women if this is unavailable, the recommended TSH upper normal limit cutoff is 4.0 mIU/L. There is some evidence supporting a decreased risk for pregnancy loss and preterm delivery for pregnant women with TSH > 4.0 mlU/L receiving LT4 therapy. LT4 treatment has been associated with better reproductive outcomes in women with SCH undergoing artificial reproductive techniques, but not in those who are attempting natural conception. Thyroid function tests need to be repeated throughout pregnancy to monitor LT4 therapy. In addition to potential harms, LT4 contributes to treatment burden.