Evaluation of Thyroid Function during Pregnancy: Have We Taken a Wrong Turn?

Abstract

Recently, the American Thyroid Association (ATA)2 issued new guidelines for the diagnosis and management of thyroid diseases during pregnancy and the postpartum period (1). The document is an excellent review of current literature relating to the assessment of thyroid status during pregnancy. While universal screening for thyroid disease before or during pregnancy remains controversial, thyroid status can be accurately screened with thyroid-stimulating hormone (TSH), and additional testing such as free thyroxine (FT4) is required only when TSH is abnormal. However, one of the recommendations of the ATA document stood out: Recommendation 3 reads, “Accurate estimation of the FT4 concentrations can also be done by calculating a FT4 index.” The FT4 index is a virtually obsolete estimation of FT4 that has been replaced by newer and more accurate FT4 immunoassays and LC-MS/MS. This recommendation is based on the panel's observation that “Current uncertainty around FT4 estimates in pregnancy has led some to question the wisdom of relying on any FT4 immunoassays during pregnancy.” Here I would like to review the source of this uncertainty and propose an alternative interpretation of the literature. ### SOURCES OF UNCERTAINTY In 1991, Roti et al. compared 10 different radioimmunoassay methods for free triiodothyronine (FT3) and FT4 (2). They observed that whereas FT4 concentrations were often lower in pregnant (third trimester) women than nonpregnant women, the TSH concentrations remained within the reference interval. These findings suggested that many of the assays available at that time for the measurement of serum free thyroid hormone concentrations did not seem adequate to evaluate the real thyroid status of pregnant women. Indeed, since then numerous studies have demonstrated that FT4 measured by …