A higher body mass index (BMI) was associated with increased deiodinase activity, which in turn was associated with higher levels of glucose during pregnancy, according to study findings published in The Journal of Clinical Endocrinology & Metabolism.

Higher BMI and/or maternal weight are risk factors for gestational diabetes, and several studies have reported an association between higher maternal weight, low free thyroxine (fT4), and gestational diabetes in euthyroid women. The study examined the associations between maternal weight, deiodinase activity, glucose, and C-peptide in a cohort of 600 randomly selected non-Hispanic white women who participated in the Hyperglycemia Adverse Pregnancy Outcomes (HAPO) study. Samples were available to measure fT4, thyrotropin, and triiodothyronine (T3).

BMI, T3/fT4 ratio, and T3 were significantly higher in participants with gestational diabetes (P =.01, P =.001, and P =.005, respectively). Regardless of gestational diabetes status, maternal BMI appeared to be associated with T3/fT4 ratio (r=.40, P <.001) and T3 (r=.34, P <.001); however, it was inversely related with fT4 (r=-.21, P <.001). The results also showed that fasting thyroid hormone levels, especially the T3/fT4 ratio, were directly associated with maternal glucose measurements (fasting, 1, and 2 hours: r=.24; P <.001) and with levels of C-peptide (fasting and 1 hour: r=.27; P <.001).

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“Our study confirms cross-sectional observations by other investigators, showing a continuous direct relationship between BMI and peripheral deiodinase activity [in] euthyroid women during pregnancy,” wrote the researchers.

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Haddow JE, Metzger BE, Lambert-Messerlian G, et al. Maternal BMI, peripheral deiodinase activity, and plasma glucose: relationships among Caucasian women in the HAPO study [published online February 11, 2019]. J Clin Endocrinol Metab. doi:10.1210/jc.2018-02328