Thyroid function in pregnancy

Where these ranges come from

Healthy pregnant women from Essen and Hamburg, Germany, sampled in 2004 and measured on Roche Elecsys analysers (about 420, 370 and 170 women in the first, second and third trimesters). Women were excluded for known or suspected thyroid disease, thyroid hormone use, TSH outside 0.1 to 10 mIU/L, or extreme free T4 or free T3 values. Each limit is a non-parametric 2.5th or 97.5th centile (95th for antibodies and thyroglobulin), with its own 95% confidence interval.

Limits to keep in mind

• Thyroid antibodies were not an exclusion criterion, so TPO-positive women are probably in the cohort. That tends to push the TSH upper limit up compared with antibody-negative cohorts.
• The source does not define trimesters by gestational week. This tool uses the conventional split: up to 13+6, 14+0 to 27+6, and 28+0 onward.
• The third trimester rests on about 170 women. Its TSH upper limit has a 95% CI of 2.78 to 7.08 mIU/L, which is too wide to lean on.
• The antibody centiles describe this cohort, positives included. Use the assay cut-offs (anti-TPO 34 IU/mL, anti-Tg 115 IU/mL) to call a result positive.
• These values apply only to Elecsys and cobas e methods. Other immunoassays give different results, particularly for free T4 in pregnancy.

Guideline context

The 2017 American Thyroid Association pregnancy guideline prefers trimester-specific ranges drawn from the local population, on the local assay, in TPO-negative women with adequate iodine intake. Where those are unavailable, it suggests an upper TSH limit of about 4.0 mIU/L.

Because free T4 immunoassays are affected by the rise in binding proteins during pregnancy, the guideline also accepts total T4 interpreted against roughly 1.5 times the non-pregnant range from about 16 weeks onward.