BACKGROUND
Thyroid hormone plays an important role in female repro- ductive health. Abnormal thyroid function can result in irregular menses, infertility, and pregnancy complica- tions. However, it is unclear whether the reverse is true, where female reproductive health conditions affect thyroid function. Changes in female reproductive hormones and immune status that occur in pregnancy may contribute to permanent changes in mother’s endocrine function after pregnancy. Studies have reported an increase in breast cancer risk and risk of type 2 diabetes after multiple pregnancies in women. Other studies have suggested that pregnancy may affect mothers’ thyroid function after pregnancy. Menopause has been associated with development in autoimmune antibodies and autoimmune thyroid disease is the most common cause of thyroid problems in the United States.

There are not many long-term studies evaluating effects of changing female reproductive status, such as pregnancy and menopause, on thyroid function. The authors of this study explored possible associations between number of pregnancies, miscarriages, and menopause and thyroid status over 12 years of follow up.

THE FULL ARTICLE TITLE
Shariatzadeh S, et al. Female reproductive system and thyroid dysfunction: findings from a 12-year follow-up in the Tehran Thyroid Study. Thyroid 2024;34(11):1424- 1434. doi: 10.1089/thy.2024.0245. PMID: 39463260.

SUMMARY OF THE STUDY
A total of 2711 Iranian women from the Tehran Thyroid Study, where they had study evaluation every 3 years for 12 years, were included. Data at the beginning of the study (baseline) were used to assess potential associations between number of pregnancies, pregnancy loss before 20 weeks of pregnancy, and menopause and the development of thyroid problems. Data from 2191 women at the end of 12-year follow up were used to evaluate how development of new thyroid dysfunction during the follow-up period was associated with number of pregnancies, pregnancy loss, and menopause.

At baseline, women with history of more than 4 pregnancies were 1.12 x more likely to have overt hypothyroidism (high blood TSH level and low FT4 level) and 1.11 x more likely to have subclinical hyperthyroidism (low TSH and normal FT4 level) compared to women who were never pregnant. Women with history of multiple pregnancy loss were 2 x more likely to have overt hyperthyroidism than women without history of pregnancy loss. Over the follow-up periods, women with history of more than 4 pregnancies were about 2 x more likely to develop new subclinical hypothyroidism, and about 6 times more likely to develop new overt hypothyroidism compared to women were never pregnant. Women with history of pregnancy were less likely to develop new overt hypothyroidism than women without history of pregnancy loss. Menopause status did not affect presence of thyroid problems as baseline or development of new thyroid problems over the follow-up periods.

WHAT ARE THE IMPLICATIONS OF THIS STUDY?
The findings of the study suggest that women’s reproductive history, including number of pregnancies and history of pregnancy loss, may play a role in development of thyroid dysfunction. On the other hand, menopausal state did not affect development of thyroid dysfunction. It is thought that some fetal tissue may transfer to mothers during pregnancy and later cause autoimmune response in mothers, which may lead to development of thyroid autoimmunity and subsequent thyroid dysfunction. However, previous studies have not clearly shown an increase in thyroid autoimmunity after pregnancy, so this may not entirely explain the findings of this study.

Currently available evidence is not enough to clearly state that prior pregnancy affects subsequent thyroid function in mothers. Further studies are needed to confirm the findings of this study and evaluate the underlying mechanism. However, it would be reasonable for clinicians to adopt a personalized approach in monitoring thyroid function, especially in women with complex pregnancy history.

— Sun Y. Lee, MD MSc

Table of Contents | PDF File for Saving and Printing