From the Journals

Thyroid markers linked to risk of gestational diabetes



Thyroid dysfunction early in pregnancy may increase risk of gestational diabetes, results of a longitudinal study suggest.

Increased levels of free triiodothyronine (fT3) and the ratio of fT3 to free thyroxine (fT4) were associated with increased risk of this common metabolic complication of pregnancy, study authors reported in the Journal of Clinical Endocrinology & Metabolism.

“To our knowledge, this is the first study to identify fT3 and the fT3:fT4 ratio measured early in pregnancy as independent risk factors of gestational diabetes,” wrote Shristi Rawal, PhD, of the National Institute of Child Health and Human Development (NICHD) , and her colleagues.

Although routine thyroid function screening during pregnancy remains controversial, Dr. Rawal and colleagues said their results support the “potential benefits” of the practice, particularly in light of other recent evidence suggesting thyroid-related adverse pregnancy outcomes.

The current case control study by Dr. Rawal and her coinvestigators included 107 women with gestational diabetes and 214 nongestational diabetes controls selected from a 12-center pregnancy cohort, which included 2,802 women aged between 18 and 40 years. The thyroid markers fT3, fT4, and thyroid-stimulating hormone (TSH) were measured at four pregnancy visits, including first trimester (weeks 10-14) and second trimester (weeks 15-26).

The fT3:fT4 ratio had the strongest association with gestational diabetes. In the second trimester measurement, women in the highest quartile had an almost 14-fold increase in risk when compared to the lowest quartile, after adjusting for potential confounders including prepregnancy body mass index and diabetes family history (adjusted odds ratio, 13.60; 95% confidence interval, 3.97-46.30), Dr. Rawal and her colleagues reported. The ratio of fT3:fT4 at the first trimester was also associated with increased risk (aOR, 8.63; 95% CI, 2.87-26.00).

Similarly, fT3 was positively associated with gestational diabetes at the first trimester (aOR, 4.25; 95% CI, 1.67-10.80) and second trimester (aOR, 3.89; 95% CI, 1.50-10.10), investigators reported.

By contrast, there was no association between fT4 or TSH and gestational diabetes, they found.

“These findings, in combination with previous evidence of thyroid-related adverse pregnancy outcomes, support the benefits of thyroid screening among pregnant women in early to mid pregnancy,” senior author Cuilin Zhang, MD, MPH, PhD, of the NICHD, said in a press statement.

Thyroid function abnormalities are relatively common in pregnant women and have been associated with obstetric complications such as pregnancy loss and premature delivery, investigators noted.

Previous evidence is sparse regarding a potential link between thyroid dysfunction and gestational diabetes. There are some prospective studies that show women with hypothyroidism have an increased incidence of gestational diabetes, Dr. Rawal and her colleagues wrote. Isolated hypothyroxinema, or normal TSH and low fT4, has also been linked to increased risk in some studies, but not in others, they added.

Support for the study came from NICHD and the American Recovery and Reinvestment Act research grants. The authors reported no conflicts of interest.

SOURCE: Rawal S et al. J Clin Endocrinol Metab. 2018 Jun 7. doi: 10.1210/jc.2017-024421.

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