Purpose: The present study aims to investigate the effect of blood sugar regulation and vitamin D levels on calcium metabolism and parathormone levels in patients with type 2 diabetes mellitus.,Material and Method: We included 132 patients with type 2 diabetes mellitus who presented to our outpatient clinic for regular check up between August 2013 and October 2013. Fasting blood glucose, HbA1c, calcium, phosphorus, magnesium, albumin, creatinine, parathormone, 25-Hydroxy vitamin D [25(OH) D], spot urinary calcium and creatinine levels were studied for each patient.,Results: Vitamin D levels were below 30 ng/mL in 96.9% (n = 128) and below 20 ng/mL in 78.7% (n = 102) of the patients included in the study. Patients with impaired blood sugar regulation (HbA1c > 10%) had lower levels of PTH, albumin and 25(OH) D levels and higher phosphorus levels compared to patients with HbA1c levels below 10% (p = 0.018, p = 0.043, p = 0.002, p = 0.01, respectively). The rates of functional hypoparathyroidism (parathormone < 65 ng/mL) in patients with vitamin D levels below 30 ng/mL and 20 ng/mL were 63.2% and 59.6%, respectively. Among the diabetic patients with vitamin D levels < 30 ng/mL, magnesium levels were significantly lower in those with functional hypoparathyroidism (parathormone < 65 ng/mL) compared to those with secondary hyperparathyroidism (p = 0.015). Comparative statistical analysis of patients with HbA1c levels above and below 10% demonstrated higher proportion of patients with functional hypoparathyroidism in the group with impaired blood sugar regulation (p = 0.035 for patients with vitamin levels below 30 ng/mL, and p = 0.031 for patients with vitamin levels below 20 ng/mL).,Discussion: Impaired blood sugar regulation leads to functional hypoparathyroidism with secondary hypomagnesemia in type 2 diabetes mellitus, as was previously described for subjects with type 1 diabetes mellitus.