First author Shuai Yang, of the International Peace Maternity and Child Health Hospital in Shanghai, and colleagues noted that although thyroid dysfunction in early pregnancy may adversely affect pregnancy outcome and the child, there have been few prospective studies to evaluate these effects.
The study’s aim was to assess the correlations between various levels of thyroid hormone in early pregnancy and the incidence of gestational diabetes mellitus (GDM).
The study enrolled 27,513 women who provided early pregnancy serum samples for analyses of thyroid function. GDM was diagnosed via a two-hour, 75-gram oral glucose tolerance test (OGTT). The mothers were then grouped and compared, according to the results.
The incidence of GDM increased with maternal age: 5.83% for women younger than 25 years old; 10.18% for women between the ages of 25 and 29; 14.95% for women between 30 and 34; and 22.40% for women 35 and older.
The prevalence of GDM also rose with an increasing prepregnancy body mass index (BMI): 9.30% for less than 18.5; 12.89% for between 18.5 and 23.9; and 24.77% for 24 or greater.
The delivery method influenced GDM as well: an 11.75% incidence for vaginal and 15.46% for Cesarean section.
Pregnant women with a family history of diabetes also had a much higher incidence of GDM compared to those without: 21.09% versus 12.92%.
Another finding of the study is that both the level of the normal thyroid-stimulating hormone (TSH) and free T4 (FT4) in early pregnancy were slightly lower in GDM women than in non-GDM women.
But the level of thyroid peroxidase antibodies (TPOAb) was the same for the two groups.
The authors evaluated the incidence of GDM in patients with different levels of FT4, TSH or TPOAb, for which they found that FT4 helps to predict GDM because the incidence of GDM gradually decreased as the level of FT4 increased. Conversely, TSH and TPOAb did not predict GDM.
These results indicate that a low level of FT4 or hypothyroxinemia is an independent risk factor for GDM.
The findings of the study are particularly applicable to China, where one survey concluded that the rate of diabetes in that country has increased from 1% in 1980 to nearly 10% in 2008.
“We found that the incidence rate of GDM is approximately 13.44%, which is significantly higher than the rate in European and American women,” the authors wrote.
Furthermore, studies show that when BMI is equivalent, Asian populations have a higher incidence of diabetes than other ethnicities, perhaps due to centripetal obesity diabetes being an independent predictive factor and abdominal and visceral fat accumulating to a greater degree in Asians than Europeans and Americans with the same waist circumference.
In addition, Asians have a higher incidence of insulin resistance, and mitochondrial dysfunction is implicated in the development of central obesity and insulin resistance.
For all these reasons, the authors believe more attention should be focused on diabetes in Asia.
One of the advantages of the study is that patients with different thyroiditis and immune system diseases were excluded, as well as patients with diabetes caused by immune disorders, because previous reports indicate that autoimmune thyroiditis is closely related to type 1 diabetes.
The study also excluded patients who were on hormone drugs (either before or during pregnancy) that might affect thyroid function, plus patients who were receiving thyroid hormone replacement therapy.
As a result, correlations between thyroid hormone levels in early pregnancy and GDM could be studied in a natural setting.
On the other hand, the study did not examine FT3 levels during early pregnancy, due to its retrospective nature.
Going forward, the investigators would like screenings during early pregnancy to determine thyroid function by combining evaluations of TSH and FT4.