Vitamin D status affects the clinical and corporal outcomes of postoperative patients who undergo a Roux-en-Y gastric bypass (RYGB). The aim of this study was to evaluate the effect of adequate vitamin D serum concentrations on thyroid hormones, body weight, blood cell count, and inflammation after an RYGB. A prospective observational study was conducted with eighty-eight patients from whom we collected blood samples before and 6 months after surgery to evaluate their levels of 25-hydroxyvitamin D 25(OH)D, thyroid hormones, and their blood cell count. Their body weight, body mass index (BMI), total weight loss, and excess weight loss were also evaluated 6 and 12 months after surgery. After 6 months, 58% of the patients achieved an adequate vitamin D nutritional status. Patients in the adequate group showed a decrease in the concentration of thyroid-stimulating hormone (TSH) (3.01 vs. 2.22 µUI/mL,p= 0.017) with lower concentrations than the inadequate group at 6 months (2.22 vs. 2.84 µUI/mL,p= 0.020). Six months after surgery, the group with vitamin D adequacy showed a significantly lower BMI compared with the inadequate group at 12 months (31.51 vs. 35.04 kg/m2,p= 0.018). An adequate vitamin D nutritional status seems to favor a significant improvement in one’s thyroid hormone levels, immune inflammatory profile, and weight loss performance after an RYGB.