Revised TSH Cutoff Values by Trimester in Pregnancy
The most current guideline-based recommendation is to use an upper limit of 4.0 mIU/L for TSH in the first trimester, with recognition that TSH naturally decreases in early pregnancy and gradually rises through subsequent trimesters. 1
Trimester-Specific TSH Reference Ranges
First Trimester
- Upper limit: 4.0 mIU/L 1
- The Korean Thyroid Association (2023) adopted this as the revised upper limit, moving away from the previously stricter 2.5 mIU/L cutoff 1
- The American Thyroid Association (2017) recommends 4.0 mIU/L when population-specific reference ranges are unavailable 2
- TSH physiologically decreases during this trimester due to hCG cross-reactivity with TSH receptors 3
Second Trimester
- Normal range: 0.72-4.22 mIU/L 3
- The American College of Physicians provides this range, though some evidence suggests maintaining TSH ≤3.0 mIU/L may optimize outcomes 3
- TSH gradually rises from first trimester levels 3
Third Trimester
- TSH continues to gradually rise through this trimester 3
- Specific upper limits follow the same physiological pattern of gradual increase from second trimester values 3
Critical Context for Application
The non-pregnant reference range of 0.45-4.5 mIU/L is inappropriate for pregnant women and will miss significant thyroid dysfunction. 4
Diagnostic Thresholds for Treatment
- TSH 4.0-10.0 mIU/L with normal free T4: Subclinical hypothyroidism 1
- TSH >10.0 mIU/L: Overt hypothyroidism regardless of free T4 level 1
- Levothyroxine treatment is recommended when TSH exceeds 4.0 mIU/L in subclinical hypothyroidism, regardless of thyroid peroxidase antibody status 1
Important Caveats
- Gestational age-specific cutoffs within the first trimester may be more appropriate than a fixed 2.5 mIU/L value, as TSH decreases continuously even within weeks 3-13 5
- The controversy regarding stricter cutoffs (2.5 mIU/L) stems from older 2011-2012 guidelines that were subsequently revised due to concerns about overdiagnosis 2