Trimester-specific reference ranges

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Because of the changes in thyroid physiology during pregnancy, the Guidelines of the American Thyroid Association (ATA) for the Diagnosis and Management of Thyroid Disease During Pregnancy and Postpartum recommend using trimester-specific reference ranges for TSH and method and trimester-specific reference ranges for serum free T4.
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
Source: Thyroid 2011; 21:1081.