CST3 and obesity due to melanocortin 4 receptor deficiency: The purpose of this study was to assess the association between severe obesity, serum creatinine, and cystatin C, and to assess the impact of substantial medical weight loss on eGFR in individuals with normal fasting glucose (NFG), impaired fasting glucose (IFG), and type 2 diabetes (T2DM) in the short-term (3-to-6 months) and long-term (2-years).