Peer Reviewed

subclinical hypothyroidism

Maternal Thyroid Function in Pregnancy and Subsequent Outcomes

A maternal serum thyroid stimulating hormone (TSH) concentration of more than 4mIU/L during pregnancy may have an impact on certain pregnancy outcomes, findings suggest.

Researchers arrived at their conclusion after performing a retrospective cohort study of 8413 pregnant women aged 18 years or older with a singleton gestation and no known thyroid disease. All women included in the study had received prenatal care at Boston Medical Center between January 1, 2013, and May 22, 2014.

In this cohort, the median TSH level was found to be 1.06mIU/L, with 130 (1.6%) women having a TSH level of more than 4mIU/L. The results of the study indicated that a maternal TSH level of more than 4mIU/L was associated with statistically significant increased risks of prematurity and neonatal respiratory distress syndrome (risk ratios [RRs] 2.17 and 2.83, respectively) compared with a TSH level of 4mIU/L or less.

The researchers also noted non-statistically significant associations between TSH levels of more than 4mIU/L and increased RRs for fetal loss, preeclampsia/eclampsia, and low birth weight.

No associations were observed between a TSH level of more than 4mIU/L and preterm labor, placental abruption, cesarean section, gestational hypertension or diabetes, or neonatal intensive care unit admission.

Endocrinology Consultant discussed the clinical implications of these findings further with study author Sun Y. Lee, MD, MSc, endocrinologist at Boston Medical Center and assistant professor of medicine at Boston University School of Medicine in Massachusetts.

Endocrinology Consultant: In your study, you and your colleagues found that a maternal serum TSH concentration above 4mIU/L in pregnancy was associated with approximately 2-fold increased risks of prematurity and respiratory distress syndrome in offspring. How might these findings inform clinical practice?

Dr Lee: The current American Thyroid Association (ATA) guidelines on thyroid disease in pregnancy recommend clinicians to consider treating a TSH level of more than 4mIU/L in pregnant women with negative thyroid peroxidase antibodies given the results of several studies demonstrating adverse pregnancy outcomes, such as those we observed in our study.  However, routine testing of TSH in pregnancy is not currently recommended. Our findings support the cutoff TSH level recommended by the ATA for consideration of treatment. We would also suggest testing TSH in women who have risk factors for hypothyroidism or symptoms suggestive of hypothyroidism in pregnancy.

Endocrinology Consultant: How can these findings aid in risk stratification among pregnant women, especially since routine screening for hypothyroidism during pregnancy is not recommended?

Dr Lee: Currently, routine screening for hypothyroidism during pregnancy is not recommended because there is a lack of studies assessing the benefit of levothyroxine treatment in subclinical hypothyroidism. My colleagues and I would recommend checking TSH in any women with prior history of premature delivery or miscarriage, family history of autoimmune thyroid disease, personal history of thyroid disease, or in those who have symptoms suggestive of hypothyroidism.

Endocrinology Consultant: What key message do you hope endocrinologists, obstetricians and gynecologists, and other clinicians who care for pregnant women take away from your study?

Dr Lee: We hope that clinicians caring for pregnant women consider possible hypothyroidism and measure TSH levels when appropriate in order to minimize potential adverse pregnancy outcomes in women with subclinical hypothyroidism during pregnancy.

Endocrinology Consultant: What are the next steps in terms of future research?

Dr Lee: Clinical trials assessing the potential benefit of levothyroxine in pregnant women with subclinical hypothyroidism would help solidify recommendations regarding screening and treatment of hypothyroidism in pregnancy.

—Christina Vogt

Lee SY, Cabral HJ, Aschengrau A, Pearce EN. Associations between maternal thyroid function in pregnancy and obstetric and perinatal outcomes [Published online December 15, 2019]. J Clin Endocrinol Metab.