ATA Guidelines Tools

Physiology, Laboratory Testing, and Iodine 6 Recommendations Table 1. Definitions of (Ab)normal Thyroid Function Tests Preconception and during fertility treatment Strength * Level # Apply the same definition of (ab)normal thyroid function tests to women planning a pregnancy as those used for the general non-pregnant population. Good Practice Statement Pregnancy Strength * Level # A lab and trimester-specific reference interval a for TSH and FT4 is suggested as the preferred standard for use during pregnancy. Conditional Low If a lab and trimester-specific reference interval for TSH is unavailable, a TSH reference interval of 0.1–4.0 mU/L can be used during the first and second trimesters. b Conditional Low If a lab and trimester-specific reference interval for FT4 is unavailable, the following options may be used to define (ab)normal FT4, with the understanding of the limitations inherent to each testing platform: • A FT4 surrogate (TT4 adjusted for gestational week c or the FT4 index d ) • The non-pregnancy FT4 reference interval e Conditional Low e concept that serum TSH concentrations typically best reflects maternal thyroid function should be emphasized during clinical decision-making. Good Practice Statement e same thyroid function test methods/assay should be used for individual patient follow-up over the course of pregnancy to reduce analytic variability. Good Practice Statement a See the text and box for the recommended methodolog y to define TSH and FT4 reference intervals. b See text for those centers where the non-pregnancy upper limit for TSH is above 4.5 mU/L. For the third trimester, the reference intervals for general non-pregnant population TSH concentrations will likely suffice. c e non-pregnancy reference interval limit for total T4 can be used before gestational week 6. For gestational weeks 7 to 16, non-pregnancy reference interval limits for total T4 can be increased by 5% per week (total T4 percentage increase in pregnancy = [gestational week – 6] x 0.05) with a maximum increase of 50% by the 16th week of pregnancy. For gestational weeks >16, a 50% increase of total T4 may be estimated. d Typically defined as: TT4 (mcg/ml) × T3 uptake (%)/100 (pregnancy reference intervals typically not available). e e assay-related underestimation of FT4 concentrations during the second half of pregnancy should be taken into account, which can lead to slightly more diagnoses of isolated hypothyroxinemia and overt hypothyroidism as compared to a laboratory and trimester-specific reference interval. * Strength of Recommendation; # Level of Evidence; Good Practice Statement.

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