Vitamin D and Parathyroid Hormone Status in Pregnancy: Effect on Insulin Sensitivity, β-cell Function, and Gestational Diabetes Mellitus

Context: Previous studies have yielded conflicting findings on the relationship between vitamin D deficiency/insufficiency and gestational diabetes mellitus (GDM). We hypothesized that PTH may be an underlying factor relevant to this potential association. Objective: This study sought to evaluate the effect of vitamin D and PTH status on insulin sensitivity, beta-cell function, and glycemia in pregnancy. Setting and Design: Five-hundred-twenty-four women underwent a glucose challenge test (GCT) and oral glucose tolerance test (OGTT) in late second/early third trimester. The GCT/OGTT identified 142 women with GDM, 94 with gestational impaired glucose tolerance, 163 with an abnormal GCT and normal OGTT, and 125 with normal GCT and OGTT. Main Outcomes: Glycemia was assessed by glucose tolerance status and area under the glucose curve (AUC(gluc)) on the OGTT. Insulin sensitivity and beta-cell function were assessed by Matsuda index and Insulin Secretion-Sensitivity Index-2 (ISSI-2), respectively. Results: There were 166 women (31.7%) with vitamin D deficiency (25-OH-D < 50 nmol/L), 178 (34%) within sufficiency(25-OH-D >= 50nmol/Land <75nmol/L), and 180(34.3%) with sufficiency(25-OH-D >= 75 nmol/L). Vitamin D status was not associated with Matsuda index, ISSI-2, AUC(gluc), or glucose tolerance status. In contrast, ISSI-2 decreased and AUC(gluc) increased across ascending tertiles of PTH (P = .06 and P = .002, respectively). Indeed, the prevalence of GDM progressively increased from the first (22.6%) to second (25.8%) to third (33.5%) tertile of PTH (P < .001). On logistic regression analyses, the third tertile of PTH was independently associated with GDM (adjusted OR = 1.82; 95% CI, 1.09-3.02; P = .022), whereas vitamin D deficiency and insufficiency were not significant predictors of GDM. Conclusions: Increased PTH, rather than vitamin D deficiency/insufficiency, is independently associated with dysglycemia in pregnancy.