Amerlex free triiodothyronine and free thyroxine levels in normal pregnancy

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Abstract
Summary. 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).