The role of vitamin D deficiency and thyroid dysfunction on blood glucose regulation in patients with type 2 diabetes mellitus: A retrospective cohort study

Abstract

Background/Aim: In patients with diabetes mellitus (DM), complications due to hyperglycemia decrease the quality of life and increase mortality. Vitamin D deficiency and thyroid dysfunction negatively affect blood glucose regulation. We aimed to demonstrate effects of treatment of vitamin D deficiency and thyroid dysfunction on blood glucose regulation. This study aimed to reduce the complications that may develop due to hyperglycemia. Methods: In this retrospective cohort study, Type 2 DM patients admitted to our clinic between 2015-2018 were reviewed from hospital registry. Patients who did not attend to their control visits for DM at the 0th, 3rd and 6th months and those with the exclusion criteria were not included. Patients who regularly attended diabetes controls at the 0th, 3rd and 6th months were determined. Among them, those with 25-hydroxy (OH) vitamin D, glycated hemoglobin (HbA1c), fasting blood glucose (FBG), postprandial blood glucose (PPG), free T4, free T3, thyroid stimulating hormone (TSH) values in the hospital registry were sought. Patients with vitamin D deficiencies and thyroid disorders who began treatment at the 0th month were finally included in the study, and the effects of vitamin D replacement treatment and thyroid dysfunction treatment on blood glucose regulation parameters at the 3rd and 6th months were examined. Results: HbA1c levels significantly decreased in Type 2 DM patients whose vitamin D levels were within normal limits at the 3rd month after receiving vitamin D replacement therapy (P=0.023). Vitamin D and HbA1c levels at the 3rd month controls were negatively correlated (r=-0.23, P=0.016, respectively). There were no significant differences in FBG and PPG levels at the 3rd month (P=0.063, P=0.361, respectively). In type 2 DM patients with hypothyroidism at the 0th month who were euthyroid at the 3rd month, there were no statistically significant differences in HbA1c, FBG and PPG (P=0.202, P=0.14, P=0.40, respectively). Six type 2 DM patients became euthyroid at the 3rd and 6th months after beginning levothyroxine treatment at the 0th month, and six patients became euthyroid at the 3rd and 6th months after hyperthyroidism treatment. Two patients had their FBG, and PPG values measured. Due to the insufficient sample size, statistical significance of differences in HbA1c, FBG and PPG levels could not be determined. Conclusion: Vitamin D replacement treatment had positive effects on blood glucose regulation in DM patients with vitamin D deficiency. The effects of vitamin D on blood glucose regulation should be evaluated by HbA1c. Thyroid dysfunctions were not sufficiently questioned during the three-month follow-up of DM patients, so its effects on blood glucose regulation could not be evaluated. Thyroid dysfunction should be questioned in the 3-month follow-up of DM patients and thyroid function tests should be requested.