Normal TSH Levels During Pregnancy

Trimester-Specific Reference Ranges

• In non-pregnant women, the normal TSH reference range is typically 0.45-4.5 mIU/L 1, 2, 3

Physiological Basis for Lower TSH in Pregnancy

• Normal pregnancy is recognized as a cause of subnormal TSH concentration with normal free T4 levels 2, 3

Monitoring Recommendations

• For pregnant women with pre-existing hypothyroidism, check TSH every 4 weeks until stable, then every trimester 4, 5

Thyroid Function During Pregnancy

Physiological Changes and Monitoring

• Normal pregnancy causes subnormal TSH concentrations with normal free T4 levels due to hCG's structural similarity to TSH, which stimulates the thyroid gland, according to the American Medical Association 6
• The use of non-pregnant TSH reference ranges of 0.45-4.5 mIU/L is inappropriate for pregnant women and will miss significant thyroid dysfunction, as noted by the American Medical Association 6