Thyroid function at the third trimester of pregnancy in a Northern French population

During pregnancy, the production rate of thyroid hormone increases when iodine intake is sufficient. However, the appropriateness of the free thyroxin (FT4) immunoassay is questionable. We have therefore evaluated prospectively the thyroid function in pregnancy and the relevance of the FT4 immunoassay.

The thyroid function of 114 pregnant, healthy Parisian women with mild iodine deficiency was studied at the third trimester of pregnancy, 55 of whom served as their own control three months after delivery, and the results were compared to North American reference values.

All French pregnant women showed an increase in thyroxin binding globulin (TBG) serum levels. FT4 levels decreased by about 30% at the third trimester of pregnancy, as compared to 10-15% in the American population. Moreover, the increase in total thyroxin (TT4) secretion represented only 27%, as compared to 50% in the American population. Linear regression model analysis showed a positive correlation between levels of TT4 and TBG, TT4 and FT4, as well as FT4 and free thyroxin index (FTI).

The hypothyroxinemia at the third trimester of pregnancy was more prominent in the Parisian population and insufficient iodine intake could be responsible for the deficient increase in TT4. It is therefore concluded that the inability of the thyroid to establish the required equilibrium could be corrected by systematic iodine supplementation before pregnancy. Finally, the strong correlation between FT4 and FTI suggests that the quality of FT4 test immunoassay is appropriate for estimating FT4 serum levels during pregnancy.