Abstract
BACKGROUND: This study explores how weight loss impacts thyroid hormone levels, particularly free T3 (FT3) and FT4, in obese individuals, given the key role these hormones play in regulating metabolism and gene expression. METHODS: A comprehensive literature search in PubMed, ISI/Web of Science, and Scopus databases identified studies examining the correlation between weight, body mass index (BMI), and thyroid hormone levels. RESULTS: The analysis revealed that BMI reduction significantly decreases FT3 levels [odds ratio (OR) = 2.31, 95% confidence interval (CI): 1.73-3.10, P < 0.01] and increases FT4 levels (OR = 0.68, 95% CI: 0.47-0.98, P = 0.04). Weight loss results in a significant reduction in FT3 (OR = 2.47, 95% CI: 1.68-3.64, P < 0.01) and thyroid-stimulating hormone (TSH) (OR = 1.76, 95% CI: 1.15-2.69, P < 0.01), as well as a significant increase in FT4 (OR = 0.69, 95% CI: 0.55-0.88, P < 0.01). Subgroup analysis indicated that changes in thyroid hormone levels were more pronounced in obese individuals who underwent diet calorie restriction compared to bariatric surgery. CONCLUSION: The study results indicate that weight reduction leads to a decrease in TSH and FT3 levels and an increase in FT4 levels. Calorie restriction through diet has a more pronounced effect on thyroid function compared to bariatric surgery. Clinicians should approach elevated TSH levels in obese patients with caution, emphasizing weight management strategies as the primary therapeutic intervention before resorting to pharmacological treatments.