Abstract
SummaryQuantitation 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.
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