What is the normal Thyroid-Stimulating Hormone (TSH) level during pregnancy?

Normal TSH Levels During Pregnancy

During pregnancy, the normal TSH reference range is lower than in non-pregnant women, with the upper limit being 2.5 mIU/L in the first trimester and 3.0 mIU/L in the second and third trimesters. This is due to physiological changes that occur during pregnancy, including the thyroid-stimulating effects of human chorionic gonadotropin (hCG).

Trimester-Specific Reference Ranges

• In non-pregnant women, the normal TSH reference range is typically 0.45-4.5 mIU/L 1
• During pregnancy, normal TSH ranges are lower:
  • First trimester: 0.1-2.5 mIU/L 2, 3
  • Second trimester: 0.2-3.0 mIU/L 4
  • Third trimester: 0.3-3.0 mIU/L 4

Physiological Basis for Lower TSH in Pregnancy

• Human chorionic gonadotropin (hCG) has structural similarity to TSH and exerts thyroid-stimulating effects 4, 5
• This hCG-mediated stimulation leads to increased thyroid hormone production and a subsequent decrease in TSH levels 5
• Normal pregnancy is recognized as a cause of subnormal TSH concentration with normal free T4 levels 1

Clinical Implications

• Women with pre-pregnancy TSH levels between 1.2-2.5 mIU/L have a 50% chance of requiring increased levothyroxine dosage during pregnancy 3
• Women with pre-pregnancy TSH <1.2 mIU/L have only a 17.2% chance of requiring dose increases 3
• Higher pregnancy loss rates (6.1% vs 3.6%) have been observed in women with first-trimester TSH between 2.5-5.0 mIU/L compared to those with TSH <2.5 mIU/L, even in thyroid antibody-negative women 2

Monitoring Recommendations

• For pregnant women with pre-existing hypothyroidism:
  • Measure TSH as soon as pregnancy is confirmed 6
  • Monitor TSH at minimum during each trimester 6
  • Maintain TSH in the trimester-specific reference range 6
  • Check TSH every 4 weeks until stable, then every trimester 1

Pitfalls and Caveats

• Using non-pregnancy reference ranges can lead to both overdiagnosis and underdiagnosis of thyroid dysfunction during pregnancy 7
• A recent study found that TSH levels beyond the accepted normal range do not necessarily reflect abnormal thyroid hormone levels during pregnancy in most cases 4
• Alternative approaches to defining reference intervals (fixed upper limit of 4.0 mU/L or subtracting 0.5 mU/L from non-pregnant upper limits) have low sensitivity (0.63-0.82) and high false discovery rates (0.11-0.35) 7
• Population- and trimester-specific reference ranges are preferred but often unavailable in clinical practice 7

Treatment Considerations

• For women with pre-existing hypothyroidism who become pregnant:
  • Increase levothyroxine dosage by 12.5-25 mcg per day if TSH is above the normal trimester-specific range 6
  • Monitor TSH every 4 weeks until stable 6
  • Return to pre-pregnancy dosage immediately after delivery 6
• For newly diagnosed hypothyroidism during pregnancy:
  • TSH ≥10 IU/L: Start levothyroxine at 1.6 mcg/kg/day 6
  • TSH <10 IU/L: Start levothyroxine at 1.0 mcg/kg/day 6