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PLoS One. 2016 Feb 24;11(2):e0149065. doi: 10.1371/journal.pone.0149065. eCollection 2016.

Free Thyroxine During Early Pregnancy and Risk for Gestational Diabetes.

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Department of Pathology and Laboratory Medicine, Division of Medical Screening and Special Testing, Women and Infants Hospital and Alpert Medical School of Brown University, Providence, Rhode Island, United States of America.
Savjani Institute for Health Research, Windham, Maine, United States of America.
Maine Medical Center Research Institute, Scarborough, Maine, United States of America.
Department of Obstetrics & Gynecology, Division of Maternal-Fetal Medicine, University College of Physicians and Surgeons, New York, New York, United States of America.
Department of Obstetrics & Gynecology, Royal College of Surgeons in Ireland, Dublin, Ireland, United Kingdom.


Several studies have now reported associations between gestational diabetes mellitus (GDM) and low free thyroxine (fT4) during the second and third trimesters, but not in the first trimester. The present study further examines relationships between low fT4, maternal weight, and GDM among women in the FaSTER (First and Second Trimester Evaluation of Risk) trial, in an effort to determine the extent to which thyroid hormones might contribute to causality. The FaSTER cohort includes 9351 singleton, euthyroid women; 272 of these women were subsequently classified as having GDM. Thyrotropin (TSH), fT4, and thyroid antibodies were measured at 11-14 weeks' gestation (first trimester) and 15-18.9 weeks' gestation (second trimester). An earlier report of this cohort documented an inverse relationship between fT4 in the second trimester and maternal weight. In the current analysis, women with GDM were significantly older (32 vs. 28 years) and weighed more (75 vs. 64.5 kg). Maternal weight and age (but not TSH) were significantly associated univariately with fT4 (dependent variable), in the order listed. Second trimester fT4 odds ratios (OR) for GDM were 2.06 [95% CI 1.37-3.09] (unadjusted); and 1.89 [95% CI 1.26-2.84] (adjusted). First trimester odds ratios were not significant: OR 1.45 [95%CI 0.97-2.16] (unadjusted) and 1.11 [95% CI 0.74-1.62] (adjusted). The second trimester fT4/GDM relationship thus appeared to strengthen as gestation progressed. In FaSTER, high maternal weight was associated with both low fT4 and a higher GDM rate in the second trimester. Peripheral deiodinase activity is known to increase with high caloric intake (represented by high weight). We speculate that weight-related low fT4 (the metabolically inactive prohormone) is a marker for deiodinase activity, serving as a substrate for conversion of fT4 to free triiodothyronine (fT3), the active hormone responsible for glucose-related metabolic activity.

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