Obesity-related thyroid reflections: from weight gain to weight loss.

Obesity and thyroid disorders are highly prevalent conditions with complex, bidirectional interactions. Thyroid hormones, particularly triiodothyronine, regulate energy expenditure, thermogenesis, and lipid metabolism, while excess adiposity can modulate hypothalamic-pituitary-thyroid (HPT) axis activity. Individuals with obesity frequently exhibit elevated thyroid-stimulating hormone (TSH) levels in the absence of true thyroid disease, suggesting an adaptive rather than pathological response. The balance of current evidence suggests that non-autoimmune hyperthyrotropinemia predominates in severe and morbid obesity, whereas autoimmune hypothyroidism may be relatively more common in mild-to-moderate obesity. Obesity also induces structural changes in the thyroid, including increased volume, hypoechogenicity, and adipocyte infiltration, which may reverse with weight loss. Weight reduction interventions - including dietary strategies, bariatric surgery, and pharmacotherapy - consistently reduce TSH with variable effects on free thyroid hormones. Emerging therapies, such as that with GLP-1 receptor agonists, may influence the HPT axis directly or indirectly through weight loss, although data remain limited. Understanding the interplay between obesity and thyroid function is important for the appropriate interpretation of thyroid function tests, differentiation of adaptive versus pathological changes, and optimization of treatment strategies in patients with excess adiposity.