BACKGROUND
Thyroid hormone is essential for normal development of the baby during pregnancy. Early on, the baby gets all of their thyroid hormone from the mother. It is clear that untreated overt hypothyroidism (low FT4 and high TSH) in the mother needs to be treated with thyroid hormone to prevent any harm to the developing baby. What is not so clear is the effect of untreated subclinical hypothyroidism (high TSH, normal FT4) has on the baby. Because of this, the need to treat mild hypothyroidism in the mother, much less screen for this during pregnancy, is less clear. Guidelines recommend screening for thyroid problems in the mother based on both major established risk factors (such as positive TPO antibodies, a marker of autoimmune thyroid disease) as well as more common characteristics, such as advanced maternal age and number of pregnancies. However, studies have shown that using these risk factors alone is likely insufficient to identify all cases of thyroid problems in the mother that my need to be treated.

This study looks at additional risk factors that may predict risk for thyroid problems in the mother than are recommended by current guidelines.

THE FULL ARTICLE TITLE
Osinga JAJ et al. Risk factors for thyroid dysfunction in pregnancy: an individual participant data metaanalysis. Thyroid 2024;34(5):646-658; doi: 10.1089/ thy.2023.0646. PMID: 38546971.

SUMMARY OF THE STUDY
This study was conducted within the Consortium on Thyroid and Pregnancy, a group of population studies with information on thyroid function in the mother during pregnancy. They looked at potential risk factors, including positive TPO or thyroglobulin (Tg) antibodies, the age of the mother, body mass index (BMI), number of pregnancies, smoking status, pregnancy by in-vitro fertilization, twin pregnancy, maternal education, and maternal iodine status.

A total of 65,559 women from 25 studies were included. The screening rate in these groups using risk factors currently recommended (age >30 years, ≥2 pregnancies, BMI ≥40) was 58%, with a detection rate for overt and subclinical hypothyroidism of 59%. Risk of overt hypothyroidism was 7 % if both TPO and Tg antibodies were positive, 3.8% if only TPO antibodies were positive, 2.4 % if only Tg antibodies were positive and 0.1% if both TPO and Tg antibodies were negative. Risk of subclinical hypothyroidism was 20 % if both TPO and Tg antibodies were positive, 14.2% if only TPO antibodies were positive, 8.1% if only Tg antibodies were positive and 2.2 % if both TPO and Tg antibodies were negative. Twin pregnancy was associated with a higher risk for overt hyperthyroidism (5.6% vs. 0.7%). None of the other risk factors assess were able to predict thyroid problems in the mother.

WHAT ARE THE IMPLICATIONS OF THIS STUDY?
These data show that thyroid antibody positivity, especially TPO antibody, was associated with a higher risk for both overt and mild hypothyroidism in the mother during pregnancy. However, other risk factors such as the age of the mother, body mass index (BMI), number of pregnancies, smoking status, pregnancy by in-vitro fertilization, twin pregnancy, maternal education, and maternal iodine status did not predict thyroid problems in the mother. This study suggests that more research into effective screening strategies would be helpful to identify mothers with thyroid problems during pregnancy.

— Alan P. Farwell, MD

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