Levothyroxine in Pregnancy

Abstract

Thyroid hormone homeostasis changes markedly during pregnancy, and first trimester-specific reference ranges for thyrotropin (thyroid-stimulating hormone, TSH) are needed to diagnose hypothyroidism. Treatment consists in levothyroxine (LT4) in this setting (triiodothyronine or desiccated thyroid preparations have no role here). Severe hypothyroidism is associated with infertility, and levels of TSH above 4.0 IU/mL signal an increased risk of adverse pregnancy outcomes. All pregnant women (and women planning a pregnancy) with overt hypothyroidism must be managed effectively with oral LT4. Thyroid autoimmunity increases the risk of adverse pregnancy outcomes and is associated with certain causes of infertility. Current European and US guidelines recommend a role for patients with subclinical hypothyroidism and thyroid autoimmunity, not least to guard against progression to overt hypothyroidism during the pregnancy. Women with hypothyroidism undergoing assisted reproduction technology to become pregnant appear to be strong candidates for LT4-based therapy.