Thyroid disease is common in pregnancy not only due to its background prevalence in females of child bearing age but also due to the profound impact that pregnancy has on the thyroid, including increased thyroid hormone production by nearly 50%, increased thyroid hormone binding and direct stimulatory effects of βHCG. In pregnancy, the commonest causes of hyperthyroidism include Graves’ disease (GD) and gestational hyperthyroidism. In iodine sufficient regions hypothyroidism is most commonly caused by Hashimoto’s thyroiditis. Other diseases that may occur in pregnant women include nodular thyroid disease and thyroid cancer.
In GD, if maternal TRAb are elevated 2-3X normal or a woman is on anti-thyroid medication, ultrasound fetal surveillance should take place after 22 weeks. A free T4 target should be used for guiding treatment. Birth defects have been associated with exposure to both carbimazole and PTU in early pregnancy (2-4%), though thought to be of a less severe nature with PTU. In general, an iodine supplement of 150ug/day is recommended during pregnancy. Controversy revolves around the high prevalence of subclinical hypothyroidism; its definition/diagnosis and whether treatment produces benefit for the pregnancy and or offspring.
Considerations when defining normal TFT reference intervals in pregnancy include the specific assay/method used, gestation at testing and local knowledge of the population studied (iodine status and ethnicity).