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

Background: 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.

Methods: 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.

Results: 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.

Conclusion: 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.