Gastric bypass surgery in morbid obesity: Influence on thyroid function tests and insulin resistance status.

Background: 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). Methods: Twenty-nine subjects with morbid obesity (BMI≥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. Results: 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). Conclusion: 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.