Thus, if the laboratory does not provide trimester-specific reference ranges for TSH (mU/L), the following reference ranges can be used:
●First trimester 0.1 to 2.5
●Second trimester 0.2 to 3.0
●Third trimester 0.3 to 3.0
Some studies report a decrease in free T4 during pregnancy, others report no change or even an increase. Direct free T4 measurements may be unreliable during pregnancy.
As an alternative, serum total T4 measurements, which are more reliable during pregnancy, can be measured to assess thyroid function [15]. When free T4 measurements appear discordant with TSH measurements, serum total T4 should be measured.Total T4 and T3 levels during pregnancy are 1.5-fold higher than in nonpregnant women due to TBG excess. Thus a normal reference range for pregnancy should be used.
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