Higher fT3 levels, fT3:fT4 ratios, and second-trimester isolated hypothyroxinemia during pregnancy are predictive markers of gestational diabetes, according to research published in the Journal of Clinical Endocrinology & Metabolism.
Shristi Rawal, PhD, from the Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland, and colleagues examined associations between fT3 and fT3:fT4 ratios and gestational diabetes as routine screening of thyroid function during pregnancy. Such screening is controversial.
In a case-control study, researchers enrolled 2334 non-obese and 468 obese women who were between 8 and 13 weeks of gestation from 12 US clinical centers.
Thyroid markers were measured during 4 visits in pregnancy, including the first and second trimester. Gestational diabetes status was taken from medical records (women with gestational diabetes n=107 vs women with non-gestational diabetes n=214).
Median fT3 levels and fT3:fT4 ratios were significantly higher during the first and second trimesters before gestational diabetes diagnosis. Adjusted odds ratio (aOR) comparing the highest and lowest quartile of fT3 was 4.25 (95% CI,1.67-10.80) at the first trimester and 3.89 (95% CI, 1.50-10.10) at the second trimester.
Isolated hypothyroxinemia during the second trimester was also significantly associated with an increased risk for gestational diabetes development (aOR 1.56 at the first trimester, aOR 2.97 at the second trimester). Neither thyroid stimulating hormone level nor fT4 were associated with an increased risk for gestational diabetes.
The results are presented with a few limitations. First, the low frequency of clinical thyroid conditions in the study sample meant the joint effect of thyroid autoimmunity status and hypothyroidism on gestational diabetes risk could not be evaluated. Second, trimester-specific ranges for thyroid hormone levels were not available and thus, researchers used reference ranges recommended by the 2018 American Thyroid Association guidelines.
“Higher fT3 levels, potentially resulting from de novo synthesis or increased deiodinase activity, may be involved in the pathophysiology of [gestational diabetes],” wrote Rawal. “Our findings…support the potential benefits of thyroid screening among pregnant women.”
Rawal S, Tsai MY, Hinkle SN, et al. A longitudinal study of thyroid markers across pregnancy and the risk of gestational diabetes [published online June 7, 2018]. J Clin Endocrinol Metab. doi: 10.1210/jc.2017-02442