Measurement of Serum Free Thyroxine by RIA in Normal Populations and During Pregnancy

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Wasana Lueprasitsakul
Pichai Lueprasitsakul
Panadda Bangprathaiya
Penpun Bhuripanyo


Serum Free thyroxine (FT4) concentrations have been variously reported as either constant or falling in pregnancy. In this study , 177 serum samples from apparently normal preggnancies were used to derive trimester related euthyroid ranges throughout pregnancy for free T4 measured by Diagnostic Products and Amersham kits. Serum free T4 concentrations as determined by Amersham kits were shown to be significantly reduced in the second and third trimester (P>0.05) . The free T4 concentrations measured by Diagnostic Products Kits fell with increasing gestational age especically in the third trimester, but the range of values were not significantly reduced from normal non-pregnant women.

The observation that serum FT4 concentrations may fall as gestation progressed , as demonstrated by both the Amersham and Diagnostic Products radioimmunoassay technique, suggests that diagnosis and management thyroid disorders in pregnancy, tricester related reference ranges for FT4 must be used in pregnancy if the test is to correctly interpreted.


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Lueprasitsakul W, Lueprasitsakul P, Bangprathaiya P, Bhuripanyo P. Measurement of Serum Free Thyroxine by RIA in Normal Populations and During Pregnancy. SRIMEDJ [Internet]. 2013 Nov. 11 [cited 2023 Feb. 7];4(2):77-82. Available from:
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