TSH Cutoff Values for Diagnosing Hypothyroidism in Pregnancy by Trimester
When population-specific reference ranges are unavailable, use 4.0 mIU/L as the upper limit across all trimesters, or alternatively use a cutoff 0.5 mIU/L below the non-pregnant reference range. 1
Trimester-Specific Values
First Trimester
- Upper limit: 4.0 mIU/L 1, 2
- Lower limit: 0.1 mIU/L 1
- This represents the most current guideline recommendation from the 2017 American Thyroid Association, which revised their previous stricter cutoff of 2.5 mIU/L 1
Second Trimester
- Upper limit: 4.0 mIU/L (using the general pregnancy cutoff when population-specific ranges unavailable) 1
- Older guidelines suggested 0.2-3.0 mIU/L, but these stricter cutoffs have been revised 1
Third Trimester
- Upper limit: 4.0 mIU/L (using the general pregnancy cutoff when population-specific ranges unavailable) 1
- Older guidelines suggested 0.3-3.0 mIU/L, but these stricter cutoffs have been revised 1
Important Clinical Context
The 4.0 mIU/L cutoff demonstrates excellent diagnostic performance with 97.4% sensitivity and 98.2% specificity for detecting subclinical hypothyroidism in the first trimester 2. Research validating population-specific reference ranges found an upper limit of 4.16 mIU/L in the first trimester, which is statistically equivalent to the 4.0 mIU/L guideline recommendation 2.
Critical Caveat
Using the older 2.5 mIU/L cutoff leads to overdiagnosis, with prevalence rates of subclinical hypothyroidism reaching 20.87% compared to only 5.92% when using 4.0 mIU/L 2, 3. This stricter threshold results in unnecessary treatment of many pregnant women 1.