Subclinical hypothyroidism: Identification and treatment in pregnancy

Key Points

Pregnancy also presents challenges in interpreting common thyroid function tests. Because TSH levels are generally lower throughout pregnancy, most notably during the first trimester when hCG levels peak,⁷ physicians should use trimester-specific reference ranges only. For example, in the critical first trimester, the range between the 2.5th and 97.5th percentiles is 0.1 mIU/L to 2.5 mIU/L compared with the nonpregnant range of 0.5 mIU/L to 5 mIU/L.

Multiple assays are available for obtaining serum FT₄ levels. These vary widely in accuracy, and many clinicians may be unaware of the assay method their laboratory uses. The most accurate method is a dialysate of serum samples using online extraction/liquid chromatography/tandem mass spectrometry (LC/MS).⁸ Unfortunately, this method is fairly expensive and not widely available. If LC/MS is not available, experts recommend using method-specific and trimester-specific reference ranges of serum FT₄. This makes it difficult to establish 1 specific reference range for FT₄. Physicians can adapt the nonpregnant total T₄ range of 5 μg/dL to 12 μg/dL or 50 nmol/L to 150 nmol/L for the second and third trimesters by multiplying the range by a factor of 1.5.