Changes in serum free thyroxine, free tri-iodothyronine and thyroid stimulating hormone reference intervals in normal term pregnant women

Summary

Quantitation of serum free thyroxine (free T₄) and free tri-iodothyronine (free T₃) concentrations in pregnancy should theoretically avoid such confounding factors as low serum albumin, pregnancy changes in non-esterified fatty acids, and changes in thyroid binding globulin. The purpose of this study was to establish reference intervals for serum free T₄ and T₃, and thyroid stimulating hormone (TSH) in 120 healthy pregnant women during the third trimester, using assays unaffected by these factors. Reference intervals were determined as the central 95th centile confidence interval using non-parametric rank number analysis. Serum free T₄ and T₃ reference intervals both decreased, and the serum TSH reference interval increased when compared with non-pregnant levels. These changes are physiological, and show the importance of using pregnancy specific reference intervals for serum free T₃ and T₄ and TSH measurement when assessing thyroid function in the pregnant patient.