en Endocrinology Endocrinology Original Article Original Article <span style="font-size:14px;"><span style="font-family:Times New Roman;"><span style="line-height:14pt"><b><i><span style="color:blue">Background</span></i></b><i><span style="color:blue">:</span></i> There are inconsistent results about the effect of gastric bypass surgery on thyroid function tests in morbidly obese subjects. The aim of this study was to investigate the changes in thyroid function tests and insulin resistance status in euthyroid morbidly obese subjects before and three months after gastric bypass surgery (GBS). </span><br> <span style="line-height:14pt"><b><i><span style="color:blue">Methods:</span></i></b> Twenty-nine subjects with morbid obesity (BMI&ge;40) were enrolled in this before-after study. Patients with known thyroid disorders or a history of thyroid ablative therapy, users of drugs that affect thyroid function, or fasting blood sugar and insulin were excluded. TSH, Free T4, total T3, fasting blood sugar and insulin level, and BMI were measured before and 3 months after GBS. Statistical analysis was performed with appropriate tests and p<0.05 was considered significant.</span><br> <span style="line-height:14pt"><b><i><span style="color:blue">Results:</span></i></b> Body mass index (BMI), insulin sensitivity index (HOMA-IR), and total T3 significantly decreased after bypass surgery (all with p<0.001) but no significant changes were seen in TSH (P=0.203) and FreeT4 (P=0.33). There was a significant negative correlation between changes in HOMA-IR and changes in FreeT4 (P=0.038, r= -0.38). There was no statistically significant correlation between the percentage of excess BMI loss (%EBMIL) and changes in T3 (P=0.66), Free T4 (P=0.92), TSH (P= 0.27), and HOMA-IR (P=0.17). </span><br> <span style="line-height:14pt"><b><i><span style="color:blue">Conclusion</span></i></b><i><span style="color:blue">:</span></i> Although significant changes can occur in BMI, insulin sensitivity index, fasting blood sugar, and T3 in short-time follow-up after bariatric surgery, significant TSH and FreeT4 changes may need longer follow-ups.</span></span></span><br> &nbsp; Gastric bypass surgery, Euthyroid, thyroid function tests, Insulin resistance. 294 298 http://caspjim.com/browse.php?a_code=A-10-3034-1&slc_lang=en&sid=1 Parvin Layegh layeghpa@mums.ac.ir 100319475328460047748 100319475328460047748 Yes Metabolic Syndrome Research Center, Mashhad University of Medical Sciences, Mashhad, Iran Razieh Ayati ayatirazieh@yahoo.com 100319475328460047749 100319475328460047749 No Department of Internal Medicine, Faculty of Medicine, Birjand University of Medical Sciences and Health Services, Birjand, Iran Ali Jangjoo jangjooa@mums.ac.ir 100319475328460047750 100319475328460047750 No Surgical Oncology Research Center, Mashhad University of Medical Sciences, Mashhad, Iran Mohammad Ali Yaghoubi yaghoubima@mums.ac.ir 100319475328460047751 100319475328460047751 No Metabolic Syndrome Research Center, Mashhad University of Medical Sciences, Mashhad, Iran