The relationship between thyroid dysfunction during pregnancy and gestational diabetes mellitus

Thyroid dysfunction and gestational diabetes (GDM) are the two most common endocrine disorders that can be observed during pregnancy. Thyroid function abnormalities can be associated with insulin resistance (IR) and changes in carbohydrate metabolism. In patients with type 1 diabetes, thyroid function is usually evaluated to rule out abnormalities within a second autoimmune disease. Patients with type 2 diabetes are tested for thyroid function in view of the associated weight gain, IR, and changes in metabolism. The question arises: Should we also look for thyroid dysfunction in patients with gestational diabetes? The aim of the study was to determine whether there are abnormalities in thyroid hormone levels in pregnant women with gestational diabetes.

A monocentric, retrospective study of the Dr Shterev Hospital electronic database was performed. We analysed the medical records of 662 pregnant women, divided in two groups - 412 with GDM and 250 with normal glucose tolerance, who gave birth in the period 2017-2019. Gestational diabetes mellitus in the study group was diagnosed with a 2-h, 75-g oral glucose tolerance test (OGTT) using the International Federation of Gynaecology and Obstetrics (FIGO) and American Diabetes Association (ADA) criteria. We analysed the mean serum concentrations of thyroid-stimulating hormone (TSH); free thyroxine (FT4), free triiodothyronine (FT3), FT3:FT4 ratio, fasting plasma glucose, age and body mass index in both groups. The groups were compared using the Mann-Whitney U-test.

In patients who developed GDM, significantly higher concentrations of TSH (p < 0.0001) and FT3 (p < 0.0001), lower concentrations of FT4 (p < 0.0001), and higher FT3:FT4 ratios (p < 0.0001) were found.

The results of this pilot retrospective series reveal that high-normal to high concentration of TSH and low-normal to low concentration of FT4 as well as high FT3:Ft4 ratio could indicate increased risk of development of GDM.