Peripheral deiodinase activity: A potential explanation for the association between maternal weight and gestational hyperglycemia

High maternal weight is known to associate with both low free thyroxine and gestational diabetes mellitus. We explore a deiodinase-related mechanism that may help explain these associations. Among 108 women receiving routine oral glucose tolerance testing for gestational diabetes mellitus, we collected biophysical data and measured free thyroxine and total triiodothyronine, using residual plasma samples. Fasting triiodothyronine/free thyroxine ratio and triiodothyronine were higher among women with gestational diabetes mellitus (p？=？0.02; p？=？0.04). The triiodothyronine/free thyroxine ratio and triiodothyronine measurements at 2？h were associated with weight (r？=？0.20, p？=？0.04; r？=？0.22, p？=？0.02); free thyroxine showed a non-significant inverse weight relationship (r？=？？0.06, p？=？0.55). Glucose at all four intervals was associated with triiodothyronine/free thyroxine ratios, and triiodothyronine at 2？h. In stepwise regression, triiodothyronine/free thyroxine ratio predicted glucose more strongly than did weight. These relationships may be explained by higher maternal weight inducing peripheral deiodinase activity, resulting in higher plasma glucose (via triiodothyronine stimulation) and thereby increasing gestational diabetes mellitus risk