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Free triiodothyronine (FT3) and free thyroxine (FT4) were measured in 159 women during normal pregnancy and compared with non-pregnancy reference ranges for these hormones. FT3 values fell from the reference level of 6.34 (SD 1.06) pmol/l to 3.87 (SD 0.54) pmol/l in the 3rd trimester; corresponding figures for FT4 were: reference 16.92 (SD 2.97) pmol/l, 3rd trimester 11.29 (SD 2.01) pmol/l. There were no significant changes in the 1st trimester; 4% and 69% of FT3 results in the 2nd and 3rd trimesters respectively fell below the reference range of mean +/- 2 SD. The corresponding findings for FT4 were 4% and 42%. FT3 correlated reasonably well with total T3 (r = 0.90) and was acceptably precise (within-batch CV 2.1% at 5.6 pmol/l, between-batch CV between 3.1% and 4.7% at six levels).
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J. Parker (1985) studied this question.
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