thyroid gut axis

The thyroid gland and the gut axis share a complex bidirectional relationship that integrates endocrine, immune, microbial, and nutritional interactions to maintain physiological homeostasis and influence disease processes, particularly autoimmune thyroid diseases (AITDs) and hypothyroidism.

The thyroid hormones (THs), primarily thyroxine (T4) and triiodothyronine (T3), are central regulators of metabolism and systemic physiological functions, including critical effects on gastrointestinal structure and function such as intestinal motility, epithelial differentiation, and barrier integrity. Conversely, the gut microbiota impacts thyroid function by modulating immune responses, nutrient absorption (notably iodine, selenium, iron, zinc, and vitamin D), and thyroid hormone metabolism through microbial enzymes and metabolites like short-chain fatty acids (SCFAs) and bile acids NICE CKSYan et al. 2024Lewandowski et al. 2026Jiang et al. 2025.

Pathophysiologically, dysbiosis—an imbalance in gut microbial composition—disrupts intestinal barrier function, leading to increased permeability and systemic translocation of microbial products such as lipopolysaccharide (LPS). This triggers Toll-like receptor 4 (TLR4) and downstream nuclear factor kappa B (NF-κB) signaling pathways, inciting a pro-inflammatory cytokine milieu (TNF-α, IL-6, IL-1β) that can impair thyroid hormone synthesis by inhibiting thyroid peroxidase activity and provoking autoimmune reactions marked by elevated thyroid antibodies (e.g., TPOAb, TgAb) Yan et al. 2024, Lewandowski et al. 2026, Jiang et al. 2025].

The gut microbiota further influences the differentiation and balance of T cell subsets, notably skewing the Th17/regulatory T cell (Treg) axis. Such immune modulation exacerbates or mitigates autoimmune thyroid pathology, including Hashimoto’s thyroiditis and Graves’ disease, shaping disease severity and progression NICE CKSYan et al. 2024Lewandowski et al. 2026. Intestinal microbiota-derived SCFAs, such as butyrate and propionate, support immune homeostasis by promoting Treg differentiation and maintaining epithelial barrier integrity, whereas dysbiosis is often characterized by depletion of these beneficial SCFA producers (e.g., Faecalibacterium prausnitzii) and enrichment of pro-inflammatory taxa NICE CKSLewandowski et al. 2026.

Thyroid hormones reciprocally regulate gut physiology by modulating motility, luminal pH, bile acid secretion, and the intestinal ecosystem, thereby influencing microbial diversity and function. Dysfunctional thyroid states alter these parameters, fostering microbial dysbiosis, which creates a feedback loop affecting thyroid health NICE CKSJiang et al. 2025Lewandowski et al. 2026Odriozola et al. 2025.

Microbial enzymatic activity, involving β-glucuronidases and sulfatases from commensal bacteria, facilitates enterohepatic circulation of thyroid hormones by deconjugating inactive metabolites back to active forms, thus influencing systemic hormone bioavailability and peripheral conversion (T4 to T3) via deiodinase regulation Jiang et al. 2025Odriozola et al. 2025.

Nutritionally, gut microbiota affects the absorption and metabolism of key micronutrients essential for thyroid hormone synthesis and antioxidant defense, including iodine and selenium. Dysbiosis may impair mineral solubility and transporter expression, exacerbating hypothyroidism and autoimmunity NICE CKSLewandowski et al. 2026Jiang et al. 2025Odriozola et al. 2025.

Emerging evidence also supports the role of thyroid–gut axis dysregulation beyond autoimmune disease, extending into metabolic adaptation during physiological stress such as exercise. The thyroid–microbiome–mitochondrial axis links microbial metabolites (SCFAs, bile acids), thyroid hormone signaling, and mitochondrial efficiency to allostatic regulation during training, recovery, and environmental stress NICE CKSOdriozola et al. 2025.

Clinical approaches emphasize addressing modifiable factors such as diet quality to support gut microbial diversity (e.g., fiber-rich, Mediterranean-style diets), correcting micronutrient deficiencies (iodine, selenium, iron, zinc, vitamin D), managing gastrointestinal comorbidities, and cautious adjunctive use of probiotics or prebiotics where warranted, although routine microbiota-targeted therapies lack definitive evidence to modify thyroid disease progression NICE CKSLewandowski et al. 2026Jiang et al. 2025.

In summary, the gut–thyroid axis represents a dynamic and multidimensional interaction wherein gut microbiota composition and function influence thyroid hormone metabolism, immune tolerance, epithelial barrier function, and micronutrient handling, while thyroid hormones modulate gastrointestinal physiology and microbial ecology. These mutually reinforcing pathways contribute to the pathogenesis and clinical expression of thyroid disorders, with ongoing research focusing on leveraging this axis for improved diagnostic and therapeutic strategies NICE CKSYan et al. 2024Lewandowski et al. 2026Jiang et al. 2025Odriozola et al. 2025.