What are normal thyroid-stimulating hormone (TSH) levels during pregnancy?

Normal Thyroid Levels During Pregnancy

Thyroid-stimulating hormone (TSH) levels during pregnancy should be maintained below 2.5 mIU/L in the first trimester, below 3.0 mIU/L in the second trimester, and below 3.0 mIU/L in the third trimester, as these trimester-specific ranges optimize pregnancy outcomes and differ substantially from non-pregnant reference ranges. 1, 2

Why Pregnancy Changes TSH Reference Ranges

• Normal non-pregnant TSH ranges (0.45-4.5 mIU/L) should never be applied during pregnancy, as this leads to missed diagnoses and inappropriate treatment decisions. 3, 1

• TSH naturally decreases during pregnancy, particularly in the first trimester, due to human chorionic gonadotropin (hCG) cross-reactivity with TSH receptors, then gradually rises through the second and third trimesters. 1

• This physiological suppression of TSH with normal free T4 levels is a recognized normal finding in pregnancy. 3

Trimester-Specific TSH Reference Ranges

First Trimester

• Upper limit: 2.5 mIU/L (lower limit: 0.1 mIU/L) 2, 4
• This is the most critical period as the fetal thyroid gland is not yet functional and depends entirely on maternal thyroid hormones for neurological development. 5

Second Trimester

• Upper limit: 3.0 mIU/L (lower limit: 0.2 mIU/L) 1, 2, 4
• The American College of Physicians specifically recommends 0.72-4.22 mIU/L for the second trimester, though maintaining TSH ≤3.0 mIU/L optimizes pregnancy outcomes. 1

Third Trimester

• Upper limit: 3.0 mIU/L (lower limit: 0.3 mIU/L) 2

Clinical Significance of These Ranges

• Even TSH levels between 2.5 and 5.0 mIU/L in the first trimester are associated with significantly increased pregnancy loss (6.1% vs 3.6% in women with TSH <2.5 mIU/L), providing strong physiological evidence for the 2.5 mIU/L upper limit. 6

• Maternal hypothyroidism, even when subclinical, increases risks of low birth weight, fetal distress, and impaired neuropsychological development in the child. 2

• Small subclinical variations in thyroid function have been associated with detrimental pregnancy outcomes including low birth weight and pregnancy loss. 4

Important Clinical Caveats

• Population-specific factors matter: Thyroid function reference intervals show substantial variation between different populations due to ethnicity, body mass index, iodine status, and assay differences. 4

• Ideally, institutions should calculate their own trimester-specific and assay-specific reference intervals rather than relying on universal cutoff concentrations. 4

• In real-world practice, many women with known hypothyroidism remain inadequately treated during pregnancy—43% have TSH values at or above 2.5 mIU/L in the first trimester despite being on treatment. 7

Monitoring for Pre-existing Hypothyroidism

• Check TSH every 4 weeks until stable, then every trimester throughout pregnancy. 3

• Thyroxine requirements are higher in pregnancy due to complex hormonal changes, so dose adjustments are typically necessary. 2