BACKGROUND
Gestational diabetes mellitus (GDM), meaning the new development of diabetes during pregnancy related to the stress of being pregnant, is common. Overall, GDM is diagnosed in between 3 and 20% of pregnancies and is associated with excessive growth of the babies. Additionally, there is an increased risk of developing obesity, diabetes, and heart disease later in life for both the affected mother and her child.

The thyroid gland’s hormones are essential regulators of the body’s metabolic systems, including how the body handles sugar. Hypothyroidism in adults is associated with resistance to insulin and altered glucose metabolism. Further, type 2 diabetes and thyroid disorders have been reported to be related. Studies have investigated correlations between thyroid disease and GDM, with varying results.

In this study, the authors conducted a large study to investigate the association between thyroid function test results in early pregnancy and the subsequent risk of developing GDM.

THE FULL ARTICLE TITLE
Huang K et al 2023 Association between maternal thyroid function in early pregnancy and gestational diabetes: A prospective cohort study. J Clin Endocrinol Metab. Epub 2023 Aug 30:dgad518. PMID: 37647889.

SUMMARY OF THE STUDY
This was a study of 26,742 pregnant women included in the China Birth Cohort Study. Women were included if they had single pregnancies, no history of thyroid disease or thyroid function–altering medication, no history of diabetes, and normal blood glucose concentrations at their first pregnancy visit. Thyroid function was assessed based on blood drawn at the first prenatal visit between 6 to 13+ weeks of pregnancy and included measurements of free thyroxine (FT4), thyroid stimulating hormone (TSH), and thyroid peroxidase antibodies (TPOAbs).

All women underwent an oral glucose tolerance test between 24 and 28 weeks of pregnancy, and the diagnosis of GDM was made in accordance with the criteria established by the American Diabetes Association.

A total of 3985 pregnant women (14.9%) were diagnosed with GDM. The risk of GDM was higher in women who were older, had a higher body-mass index (BMI) prior to getting pregnant, or were smokers. Further, more women with GDM had prior pregnancies and more had achieved pregnancy by assisted reproductive technology (ie in-vitro fertilization). Thyroid function test results showed higher average TSH (1.60 vs 1.52 mIU/L), and lower average FT4 (16.27 vs. 16.38 pmol/L) concentrations in women with GDM than in those without GDM. Also, more women with GDM were TPOAb-positive than those without GDM (11.3% vs. 12.5%). Overall, the occurrence of GDM was significantly associated with TSH but not with FT4 or TPOAb positivity. GDM risk increased significantly with higher TSH in the group of women with a TSH ≤1.24 mIU/L, but not in those with TSH >1.24 mIU/L.

WHAT ARE THE IMPLICATIONS OF THIS STUDY?
In a large study of pregnant women with no history of thyroid disease or diabetes, there might be an association between a high normal TSH in the first trimester and GDM assessed by oral glucose-tolerance test midpregnancy. This was mainly the case within a subset of women with low concentrations of TSH. There was no clear association with subclinical hypothyroidism. Other risk factors for GDM included higher maternal age, higher prepregnancy BMI, conception by assisted reproductive technology, and smoking. Importantly, the TSH levels were all in the normal range and this does not address whether the risk of GDM could be limited by treatment with thyroid hormone.

—Alan P. Farwell, MD

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