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Box 1. Preferred Definitions of Thyroid Function Test
Abnormalities During Pregnancy
• A laboratory and trimester-specific reference interval is optimally defined as the
2.5th–97.5th percentile range of a population representative of a healthcare provider's
patient population that only includes women without major thyroid interfering
factors (e.g., TPOAb negative women with a singleton pregnancy, no known thyroid
disease, and no [regional] severe iodine deficiency).
• Various other factors that may be associated with higher mean TSH and free
thyroxine FT4 concentrations (e.g., comorbidities, assisted reproductive technolog y,
or thyroglobulin antibody [TgAb] positivity) have not been shown to substantially
change the reference interval limits for TSH or FT4 during pregnancy. However,
removal of additional cases from a study population could impact the quality of the
reference intervals by lowering the number of included individuals. Thus, it does not
seem necessary to use these additional exclusion criteria.
• Thyroid dysfunction during pregnancy is defined as follows:
▶ Overt primary hypothyroidism: increased (>96.5th percentile) TSH
concentration with a decreased (<2.5th percentile) FT4 concentration.
▶ Subclinical hypothyroidism: increased TSH concentration with a normal
(2.5th–97.5th percentile) FT concentration.
▶ Isolated maternal hypothyroxinemia: decreased FT4 concentration with a normal
concentration.
▶ Overt hyperthyroidism: decreased TSH concentration with elevated FT4 and/or
(F)T3 concentration.
▶ Subclinical hyperthyroidism: decreased TSH concentration with normal FT4
and (F)T3 concentration.
