What are the normal Thyroid-Stimulating Hormone (TSH) levels for a pregnant individual by trimester?

Normal TSH Levels by Trimester of Pregnancy

Pregnant women should use trimester-specific TSH reference ranges, with upper limits of ≤2.5 mIU/L in the first trimester and ≤3.0 mIU/L in the second and third trimesters, as standard non-pregnant ranges do not apply during pregnancy. 1

Trimester-Specific TSH Reference Ranges

First Trimester (9-13 weeks)

• Upper limit: 2.5 mIU/L is the treatment threshold recommended for pregnant women 1
• Research-based reference range: 0.08-3.29 mIU/L (2.5th-97.5th percentile) 2
• Alternative research range: 0.07-3.96 mIU/L 3
• TSH naturally decreases during the first trimester due to hCG cross-reactivity with TSH receptors 4

Second Trimester (24-28 weeks)

• Upper limit: 3.0 mIU/L is the treatment threshold for this trimester 1
• Research-based reference range: 0.59-4.22 mIU/L 2
• Alternative research range: 0.27-4.53 mIU/L 3
• TSH gradually rises from first trimester levels as hCG effects diminish 4

Third Trimester (32-36 weeks)

• Upper limit: 3.0 mIU/L is the treatment threshold for this trimester 1
• Research-based reference range: 0.81-4.33 mIU/L 2
• Alternative research range: 0.48-5.40 mIU/L 3
• TSH continues to rise progressively through the third trimester 5

Critical Clinical Context

Normal non-pregnant reference ranges (0.45-4.5 mIU/L) must never be applied during pregnancy, as this leads to missed diagnoses or inappropriate treatment. 4

• TSH levels show continuous physiological changes throughout pregnancy, with the lowest values in the first trimester and progressive increases thereafter 4, 6
• Women with TSH levels between 2.5-5.0 mIU/L in the first trimester (even if thyroid antibody-negative) have significantly higher pregnancy loss rates (6.1% vs 3.6%) compared to those with TSH <2.5 mIU/L 7
• Intra-individual TSH variability occurs naturally during early pregnancy, with levels showing continuous, uniform decreases in women who initially have values slightly above 2.5 mIU/L 6

Important Caveats

• These ranges are treatment thresholds, not necessarily the full physiological reference ranges observed in research studies 1, 2, 3
• Regional and assay-specific variations exist, making it advisable to use locally-established reference ranges when available 2, 3
• Pregnancy itself interferes with normal TSH levels, and TSH can vary by up to 50% day-to-day in the same individual 1
• Iodine supplementation status affects TSH levels, with pregnant women using iodized salt showing greater TSH decreases during early pregnancy 6