Metabolic Surgery as a Modulator of the Thyroid–Gut Axis: A Narrative Review on Autoimmunity, Function, and Levothyroxine Pharmacokinetics

Background: The interplay between obesity and thyroid dysfunction is complex, characterized by adaptive hyperthyrotropinemia and peripheral hormone resistance. Metabolic and Bariatric surgery (MBS) has emerged not only as a weight-loss (WL) intervention but also as a potent modulator of the thyroid–gut axis. Methods: We conducted a narrative review of the literature (2015–2025), synthesizing data from prospective cohorts, meta-analyses, and mechanistic studies to evaluate the impact of MBS on thyroid function, autoimmune dynamics, and drug pharmacokinetics. Discussion: Current evidence suggests that MBS promotes a recalibration of the thyroid axis. Post-operative WL is independently associated with a significant reduction in serum thyroid-stimulating hormone (TSH) and free triiodothyronine (fT3) levels, reversing obesity-induced peripheral resistance. Concurrently, the reduction in systemic inflammation (NOD-like receptor protein 3 (NLRP3) inflammasome deactivation) may dampen lymphocytic infiltration, while the amelioration of gut dysbiosis and intestinal permeability is hypothesized to reduce cross-reactivity mechanisms (molecular mimicry), leading to decreased antibody titers in Hashimoto’s thyroiditis. However, these benefits are counterbalanced by altered drug absorption mechanisms. While most hypothyroid patients benefit from reduced Levothyroxine (L-T4) requirements due to decreased lean mass, malabsorptive procedures (Roux-en-Y Gastric Bypass, One Anastomosis Gastric Bypass) can precipitate refractory hypothyroidism due to bypassed absorptive surfaces and altered gastric pH. Conclusions: MBS offers a dual benefit of functional restoration and modulation of autoimmune markers. However, post-surgical management requires a tailored approach. Clinicians must distinguish between the physiological decline in TSH (adaptive) and iatrogenic malabsorption, advocating for liquid L-T4 formulations in complex malabsorptive phenotypes.