Normal TSH Range in Second Trimester of Pregnancy
The normal TSH range for the second trimester of pregnancy is 0.2-3.0 mIU/L according to older guidelines, though the 2017 American Thyroid Association now recommends using either population-specific reference ranges or a less restrictive upper limit of 4.0 mIU/L when local data are unavailable. 1, 2
Evolution of TSH Reference Ranges in Pregnancy
The controversy surrounding TSH cutoffs in pregnancy has undergone significant evolution:
Historical stricter guidelines (2011-2012): The American Thyroid Association and Endocrine Society initially recommended TSH ranges of 0.1-2.5 mIU/L (first trimester), 0.2-3.0 mIU/L (second trimester), and 0.3-3.0 mIU/L (third trimester). 1, 2
Revised 2017 recommendations: Due to concerns about overdiagnosis and unnecessary treatment, the American Thyroid Association revised guidelines to recommend an upper limit 0.5 mIU/L below the patient's preconception TSH value, or 4.0 mIU/L when population-specific ranges are unavailable. 1
Why TSH Reference Ranges Differ in Pregnancy
Physiological changes during pregnancy necessitate trimester-specific reference ranges:
Human chorionic gonadotropin (hCG) has TSH-like effects on the thyroid gland, particularly in early pregnancy, causing physiological TSH suppression. 3
TSH levels vary significantly by gestational week even within the first trimester—up to week 6, TSH maintains non-pregnant ranges, but during weeks 9-12, TSH drops approximately 0.4 mIU/L below non-pregnancy upper limits. 2
The lower limit remains relatively stable at 0.1 mIU/L throughout the first trimester. 2
Critical Considerations for Clinical Practice
Population-specific factors significantly affect reference ranges:
Substantial variation exists between different populations due to ethnicity, body mass index, iodine status, and assay methodology. 4
International guidelines recommend calculating trimester- and assay-specific reference intervals for each institution rather than relying on universal cutoffs. 4
When institution-specific ranges are unavailable, using 4.0 mIU/L as the upper limit for second trimester is more appropriate than the older 3.0 mIU/L cutoff to avoid overdiagnosis. 1
Important Caveats About TSH Screening in Pregnancy
TSH may not reliably reflect thyroid hormone status during pregnancy:
In one large study, only 12.8% of pregnant women with TSH >10 mIU/L actually had low FT4 levels in the first trimester, suggesting TSH elevation alone may not indicate true hypothyroidism. 3
The thyrotropic effects of hCG may explain why TSH levels beyond accepted normal ranges often do not correlate with abnormal thyroid hormone levels, particularly in the first trimester. 3
Management Implications for Second Trimester
For women already on levothyroxine:
Target TSH should be maintained in the normal range using the lowest effective dose, with free T4 in the high-normal range. 5
Levothyroxine requirements typically increase 25-50% during pregnancy, necessitating proactive dose adjustments. 5
Monitor TSH every 4-6 weeks during pregnancy to ensure adequate replacement. 5
For newly diagnosed cases:
Treatment is clearly indicated when TSH exceeds 10 mIU/L with low free T4 (overt hypothyroidism). 5
For subclinical hypothyroidism (elevated TSH with normal free T4), treatment decisions should consider the degree of TSH elevation, presence of thyroid antibodies, and symptoms. 1
Inadequate treatment carries risks including preeclampsia, low birth weight, and potential neurodevelopmental effects in offspring. 5
Common Pitfalls to Avoid
Using non-pregnancy reference ranges: This leads to overdiagnosis and unnecessary treatment, as pregnancy physiology naturally lowers TSH. 4
Applying uniform first-trimester limits throughout: TSH varies significantly by gestational week, and using fixed cutoffs may cause misclassification. 2
Ignoring population-specific factors: Ethnicity, BMI, and iodine status substantially affect reference ranges, making universal cutoffs problematic. 4
Treating based on TSH alone without checking free T4: TSH elevation during pregnancy may not reflect true hypothyroidism due to hCG effects. 3