Clinical Thyroidology® for the Public

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Having low thyroid hormone level in early pregnancy may be associated with early birth of the baby

CTFP Volume 14 Issue 4

Thyroid hormone is essential for normal growth and development of the baby during pregnancy. During early pregnancy the mother makes all the thyroid hormone for the baby. Near the end of the 1st trimester, the baby’s thyroid develops and begins to make thyroid hormone. For the remainder of the pregnancy, thyroid hormone from both the mother and the baby help the baby to develop normally. Lack of thyroid hormone during pregnancy, either from the mother or the baby, can cause problems with brain development and poor pregnancy outcomes, including early birth of the baby.

Hypothyroidism is diagnosed when thyroid hormone level is low and TSH is elevated. Mild/subclinical hypothyroidism is diagnosed when thyroid hormone level is normal and TSH is elevated. During pregnancy, the mother’s thyroid hormone level may be low but her TSH is normal, a condition known as isolated maternal hypothyroxinemia (IMH). While it is clear that hypothyroidism can cause problems with the baby, we do not know whether IMH has any bad effect on the baby or the pregnancy. This study was done to find out whether there was an association between having IMH in early pregnancy and early birth of the baby.

Yang X et al 2020 The association between isolated maternal hypothyroxinemia in early pregnancy and preterm birth. Thyroid 30:1724–1731. PMID: 32434441.

The study was done in Shanghai, China between January 2013 and December 2016. IMH was defined as a TSH within normal range and a low free T4. Preterm (early) birth was defined as birth of the baby before 37 weeks of pregnancy. The researchers excluded the women who had a twin pregnancy, experienced a miscarriage, used a medication that could affect the thyroid function, or had a history of thyroid disease.

There were 41,911 patients in the study, 963 were diagnosed with IMH and 40,948 had normal thyroid function. The patients who had IMH were older, weighed more and had higher education level. There were also more women with multiple past pregnancies, diabetes mellitus during pregnancy and positive TPO antibodies in the IMH group.

Women with IMH had an increased risk of having their baby earlier than 37 weeks. An interesting finding was that the risk was higher if the baby was a girl.

In conclusion, this study found that IMH in early pregnancy may be associated with early birth of the baby and the baby’s sex may also affect the risk. We need further studies to find out whether treatment with thyroid hormone would benefit these patients and their babies.

— Ebru Sulanc, MD


Hypothyroidism: a condition where the thyroid gland is underactive and doesn’t produce enough thyroid hormone. Treatment requires taking thyroid hormone pills.

Thyroxine (T4): the major hormone produced by the thyroid gland. T4 gets converted to the active hormone T3 in various tissues in the body.

TSH: thyroid stimulating hormone — produced by the pituitary gland that regulates thyroid function; also the best screening test to determine if the thyroid is functioning normally.