Normal Thyroid Function Tests in First Trimester of Pregnancy
In the first trimester of pregnancy, TSH should be 0.1-2.5 mIU/L, which is lower than non-pregnant reference ranges due to physiological changes driven by hCG stimulation of the thyroid gland. 1
Trimester-Specific TSH Reference Ranges
- First trimester TSH: 0.1-2.5 mIU/L is the internationally recommended reference interval when institution-specific ranges are unavailable 1
- Second trimester TSH extends to 0.2-3.0 mIU/L, reflecting the physiological rise as pregnancy progresses 1
- TSH progressively increases throughout pregnancy, with mean values rising from approximately 1.2 µIU/mL in the first trimester to 3.3 µIU/mL by the third trimester 2
Free T4 (FT4) Reference Ranges
- FT4 levels are typically elevated or high-normal in the first trimester compared to non-pregnant values 3
- Mean FT4 in first trimester ranges approximately 1.01 ± 0.15 ng/dL in population studies 4
- FT4 values show significant inter-assay variability, making laboratory-specific reference ranges essential 5
Free T3 (FT3) Reference Ranges
- Mean FT3 in first trimester is approximately 4.50 ± 0.64 pmol/L 4
- FT3 levels rise from first to second trimester (mean 1.85 to 2.47 nmol/L), then decline in third trimester 2
Critical Clinical Considerations
Every institution should ideally calculate its own trimester-specific and assay-specific reference intervals rather than relying on universal cutoffs. 1
Why Institution-Specific Ranges Matter:
- Population-specific factors including ethnicity, body mass index, iodine status, and geographic location significantly affect thyroid function test results 1
- Inter-assay correlation coefficients vary widely: 0.908-0.975 for TSH, 0.676-0.892 for FT4, and only 0.480-0.789 for FT3 5
- Pregnancy alters thyroid hormone binding protein concentrations, causing immunoassays to yield different results depending on the methodology used 5
Physiological Changes Explaining Lower TSH:
- Human chorionic gonadotropin (hCG) peaks in the first trimester and has structural similarity to TSH, causing direct thyroid stimulation 1
- This hCG-mediated stimulation suppresses TSH while maintaining or elevating FT4 levels 3
- TSH levels show continuous, uniform decrease during the first half of pregnancy in healthy women 6
Common Pitfalls to Avoid
- Do not use non-pregnant reference ranges for TSH (typically 0.4-4.0 mIU/L), as this will miss subclinical hypothyroidism in pregnancy 1
- Do not assume universal cutoffs apply to all populations, as substantial variation exists between different ethnic groups and iodine-sufficient versus iodine-deficient regions 1
- Recognize that even subclinical variations in thyroid function (TSH >2.5 mIU/L in first trimester) are associated with adverse outcomes including low birth weight and pregnancy loss 1
- Women with TSH >2.5 mIU/L in early pregnancy often show spontaneous TSH decline during the first half of pregnancy, particularly with adequate iodine supplementation 6
Monitoring Implications
- For women with diagnosed hypothyroidism on levothyroxine, adjust dosage every 4 weeks until TSH is stable, then check TSH every trimester 3
- For hyperthyroidism treatment, monitor FT4 or FTI every 2-4 weeks during active treatment until stable 7
- Maintain FT4 in the high-normal range when treating hyperthyroidism to avoid fetal hypothyroidism from overtreatment 7