IntroductionPre-operative Vitamin D deficiency is markedly prevalent in prospective bariatric surgery patients. While bariatric surgery leads to significant weight loss, it can exacerbate or prolong Vitamin D deficiency. We systematically reviewed the literature to assess whether secondary hyperparathyroidism is maintained in the medium to long term in patients following the Roux-en-Y gastric bypass. MethodsA comprehensive literature search was conducted through Medline, Embase, Scopus, Web of Science, Dare, Cochrane library and HTA database. The search terms used were bariatric surgery, gastric bypass and hyperparathyroidism. ResultsFourteen studies were included (n=2688 subjects). Parathyroid hormone levels rose gradually from a mean pre-operative level of 5.691.2pmol/L to 6.36 +/- 0.77pmol/L, 7.59 +/- 0.73pmol/L and 8.29 +/- 1.41pmol/L at 2years, between 2 and 5years, and beyond 5years, respectively. Vitamin D levels slowly fell to a mean of 20.50 +/- 4.37ng/mL and 20.76 +/- 3.80ng/mL between follow-up intervals 2-5years and beyond 5, respectively. ConclusionIt appears that hyperparathyroidism persists at 5-year follow-up after gastric bypass, despite most patients being supplemented with calcium and Vitamin D.