Thyroid Function in Pregnancy

TSH Monitoring and Interpretation

The American Academy of Family Physicians recommends TSH testing as the initial test for screening and evaluating pregnant patients with suspected thyroid disease, with both TSH and free T4 (FT4) or free thyroxine index (FTI) performed in pregnant women with suspected hyperthyroidism or hypothyroidism 1, 2
In hyperthyroidism, TSH is suppressed with elevated FT4 or FTI, while in hypothyroidism, TSH is elevated, with subclinical hypothyroidism identified by an elevated TSH level in a pregnant woman without symptoms 1, 3, 4
The American Thyroid Association guidelines recommend an upper cutoff limit 0.5 mIU/L less than the preconception TSH value, or 4.0 mIU/L when local population-specific reference ranges are not available, for interpreting TSH levels in pregnant women 1, 2

Hyperemesis gravidarum is associated with biochemical hyperthyroidism, including undetectable TSH levels and elevated FTI, due to high hCG levels during the first trimester, and rarely requires treatment 5, 6
Pre-existing hypothyroidism often requires a 25-50% increase in levothyroxine dosage during the first trimester, with TSH monitored every trimester and dosage adjusted to maintain TSH in the normal range 2, 7

Failure to use trimester-specific TSH reference ranges can lead to misinterpretation of thyroid function tests, underdiagnosis of hypothyroidism, and serious maternal and fetal consequences, including preeclampsia, low birth weight, and impaired fetal neurodevelopment 1, 4