Relationship between Vitamin D Level and Subclinical Hypothyroidism in Patients with Gestational Diabetes Mellitus

The aim was to investigate the relationship between the vitamin D (VitD) level and the incidence rate of subclinical hypothyroidism in patients with gestational diabetes mellitus (GDM) in early pregnancy.

Ninety eligible patients who underwent physical examination from March 2015 to September 2017 were selected. The levels of serum 25-hydroxyvitamin D (25-OH-VD), free thyroxine (FT4), free triiodothyronine (FT3), thyroid stimulating hormone (TSH), and thyroid peroxidase antibody (TPOAb) were measured. The correlations of VitD level with those of thyroid function-related indices were analyzed.

The serum 25-OH-VD level was (17.82 ± 3.45) ng/mL in the 90 GDM patients in early pregnancy, including 64.44% patients in VitD deficiency group, 22.22% patients in VitD insufficiency group, and 13.33% in VitD sufficiency group. No significant differences in the levels of serum FT4, FT3, and TSH were found among different VitD groups (p > 0.05), but the level of TPOAb gradually decreased (p < 0.05). There was no significant difference in the incidence rate of subclinical hypothyroidism among different VitD groups (p > 0.05). VitD level was not correlated with those of FT4, FT3, and TSH (p > 0.05), but negatively correlated with that of TPOAb (p < 0.05). An increase of TPOAb level raised the risk of VitD deficiency in pregnant women (p < 0.05). Patients suffered from significant 25-OH-VD deficiency in spring and winter (p < 0.05).

GDM patients in early pregnancy display VitD deficiency, and their serum 25-OH-VD level is affected by seasons and not significantly related to subclinical hypothyroidism. However, the increased level of TPOAb may raise the risk of VitD deficiency in pregnant women.