Hypothyroxinemia and Pregnancy.

Abstract:

Objective: To evaluate the peer-reviewed literature on iodine deficiency and hypothyroxinemia in pregnancy.

Methods: Analysis of the following: published studies on isolated hypothyroxinemia in pregnancy, methodology of free T4 assays, impact of iodine deficiency on free T4 levels, and status of ongoing prospective randomized trials of isolated hypothyroxinemia during pregnancy.

Results: Studies reveal that hypothyroxinemia during pregnancy is common; they have demonstrated the pivotal role exerted by maternal thyroxine on fetal brain development and the negative impact of hypothyroxinemia on neurobehavioral performance in offspring. Two intervention studies have demonstrated a positive effect on child neurodevelopment of mothers promptly supplemented with iodine when compared with non-supplemented mothers. Free T4 assays presently in clinical use have limitations. Preliminary results of the Controlled Antenatal Thyroid Study (CATS) reveal somewhat mixed findings and the NIH-Maternal Fetal Medicine TSH Study (TSH Study) will be completed in 2015.

Conclusions: Knowledge regarding the impact of isolated hypothyroxinemia has progressed, but major questions remain. An optimal diagnostic test for free T4 during pregnancy (accurate, inexpensive, and widely available) remains elusive. Trimester-specific normative data and normal ranges from different areas do not exist. Finally analysis of the CATS trial, and results of the NIH 2015 TSH Study should yield much needed data on the impact of treating isolated hypothyroxinemia of pregnancy on the IQ of the offspring. Data published to date are insufficient to recommend levothyroxine therapy in pregnant women with isolated hypothyroxinemia. An adequate amount of iodine intake has to be recommended pre-conception or starting early in pregnancy.