Outcomes of Bariatric Surgery in Hypothyroid Patients Taking Levothyroxine: A Systematic Review and Meta-Analysis

Obesity is among the most serious challenges faced by global public health. Because of the growing rate of obesity, the number of bariatric surgeries as an effective treatment is increasing day by day. Bariatric surgeries lead to significant anatomical and physiological changes in the gastrointestinal tract. Oral levothyroxine has been the treatment of choice for hypothyroidism for many years and is a narrow therapeutic index (NTI) drug, and its absorption can be significantly influenced by alterations in gastrointestinal physiology and anatomy. The aim of this study was to evaluate the impact of bariatric surgery on levothyroxine requirements and to assess changes in thyroid function tests. A literature search was conducted using Medical Subject Headings (MeSH) terms and relevant keywords related to surgery, bariatric procedures, hypothyroidism, and levothyroxine across PubMed, Embase, Cochrane, and ClinicalTrials.gov from inception to May 2025. Cohort studies comparing levothyroxine dose and thyroid function tests before and after bariatric surgery were included. A random-effects model using the inverse variance method was applied to account for clinical and methodological heterogeneity. Standardized mean differences (SMDs) with 95% confidence intervals (CIs) were calculated. Heterogeneity was assessed using the Chi-square (χ²) test, I² statistic, and Tau² for between-study variance. The pooled effect was tested using a Z-test, with significance set at P < 0.05. Seventeen cohort studies with 922 patients were included in this meta-analysis. This study showed a mild and non-significant reduction in the dose of levothyroxine after bariatric surgery (SMD of 0.19 (95% CI:-0.05 to 0.42, p = 0.1) and I² = 84%) and indicated a very small and statistically non-significant improvement in favor of post-operative thyroid-stimulating hormone (TSH) (SMD is 0.03 (95% CI: -0.20 to 0.27), P=0.78, I² = 80%). There was a statistical improvement in triiodothyronine (T3) levels (SMD = 0.60; 95% CI: 0.03 to 1.17; p = 0.04 and I² = 91%) and no significant difference in T4 (SMD = 0.01; 95% CI: -0.58 to 0.60; p = 0.97; I² = 80%) levels after surgery. This meta-analysis suggests bariatric surgery may modestly reduce levothyroxine dose and improve T3 levels in hypothyroid patients, with no significant changes in TSH or thyroxine (T4). High heterogeneity and limited study quality highlight the need for individualized dose adjustment and further robust research.