Normal TSH in 2nd Trimester
The normal TSH range in the second trimester of pregnancy is 0.72-4.22 mIU/L based on the most recent trimester-specific reference data, though some evidence suggests maintaining TSH ≤3.0 mIU/L may optimize pregnancy outcomes.
Trimester-Specific Reference Ranges
The most recent 2024 study establishing trimester-specific reference intervals found second trimester TSH values of 0.72-4.22 mIU/L in a relevant pregnant population 1. This represents the most current data using appropriate methodology for defining pregnancy-specific ranges.
An earlier 2003 study from an iodine-deficient region reported a broader second trimester range of 0.1-5.5 microIU/ml (equivalent to mIU/L), with a mean TSH of 2.12 microIU/ml 2. However, this study acknowledged that values differ in iodine-deficient areas and may not be universally applicable.
Important Clinical Context
Why Pregnancy-Specific Ranges Matter
- TSH naturally decreases during pregnancy, particularly in the first trimester due to hCG cross-reactivity with TSH receptors, then gradually rises through the second and third trimesters 3, 2
- Normal non-pregnant reference ranges (0.45-4.5 mIU/L) do not apply during pregnancy and can lead to missed diagnoses or inappropriate treatment 3, 1
- Intra-individual TSH variability exists, with TSH levels showing continuous, uniform decrease during the first half of pregnancy, particularly in women using iodized salt 4
Treatment Thresholds vs. Normal Ranges
While the upper limit of normal may extend to 4.22 mIU/L in the second trimester 1, treatment guidelines recommend different targets:
- Recent guidelines suggest TSH target levels of ≤3.0 mIU/L for the second and third trimesters when treating hypothyroidism 5
- Evidence shows increased pregnancy loss rates in thyroid antibody-negative women with first trimester TSH between 2.5-5.0 mIU/L compared to those with TSH <2.5 mIU/L (6.1% vs 3.6%, p=0.006) 6
- When treating newly diagnosed subclinical hypothyroidism in pregnancy, the goal is to achieve TSH ≤3.0 mIU/L in the second and third trimesters 5
Clinical Pitfalls to Avoid
- Do not apply non-pregnant reference ranges to pregnant women, as this will miss cases of subclinical hypothyroidism that may impact pregnancy outcomes 1, 2
- Consider population-specific factors including ethnicity, iodine prophylaxis status, and regional differences when interpreting TSH values 1
- Recognize that "normal" statistical ranges differ from optimal treatment targets - a TSH of 4.0 mIU/L may be statistically normal but suboptimal for pregnancy outcomes 5, 6
- Account for timing of measurement as TSH continues to evolve throughout pregnancy, with progressive increases from first to third trimester 2