Maternal First Trimester TSH Concentrations: Do They Affect Perinatal and Endocrine Outcomes?

Area of Science:

• Endocrinology
• Reproductive Medicine
• Obstetrics

Background:

• Thyroid-stimulating hormone (TSH) levels naturally fluctuate during the first trimester of pregnancy.
• Understanding the distribution and clinical significance of early pregnancy TSH is crucial for maternal and fetal health.
• Previous research has not fully elucidated the association between low first trimester TSH and long-term maternal thyroid status.

Purpose of the Study:

• To investigate the distribution of first trimester TSH levels in pregnant women without pre-existing thyroid disease.
• To evaluate the association between low first trimester TSH concentrations and perinatal outcomes.
• To assess the risk of developing maternal thyrotoxicosis in relation to first trimester TSH levels.

Main Methods:

• Retrospective cohort study of 13,841 women delivering singleton infants between 2001-2011.
• Exclusion of women with prior thyroid disease; inclusion of those with first trimester TSH < 4.0 mU/l.
• Women categorized into quartiles and specific low TSH groups (TSH <0.1, 0.11-0.2, 0.21-0.4, 0.4-4 mU/l) for analysis.

Main Results:

• Mean first trimester TSH decreased from 2.09 mU/l at 5 weeks to 1.29 mU/l at 8-9 weeks.
• The lowest TSH quartile showed a 3.64-fold increased odds of future thyrotoxicosis compared to the highest quartile.
• TSH <0.1 mU/l was associated with a 10.03-fold increased odds of future thyrotoxicosis compared to TSH 0.41-4 mU/l.
• No significant association found between low TSH levels and adverse pregnancy outcomes or fetal sex.
• A small percentage of women with low first trimester TSH developed subsequent thyrotoxicosis.

Conclusions:

• Low maternal first trimester TSH concentrations are not associated with adverse pregnancy outcomes.
• Severely suppressed TSH (<0.1 mU/l) in early pregnancy is a significant risk factor for developing maternal thyrotoxicosis.
• Monitoring and further investigation may be warranted for pregnant women with very low TSH levels in the first trimester.