Protective effect of obeticholic acid on cyclophosphamide-induced thyroid toxicity in rats by inhibiting TXNIP/NLRP3/ASC/caspase-1-dependent pyroptosis and p53/BAX/caspase-3-dependent apoptosis

Cyclophosphamide (CYC) is an alkylating agent that is widely used in cancer chemotherapy and immunosuppressive therapy; however, its clinical application is limited because it causes multiple organ toxicities, including thyroid dysfunction. Obeticholic acid (OCA), a farnesoid X receptor (FXR) agonist, possesses antioxidant, anti-inflammatory and anti-apoptotic properties, suggesting potential protective effects. This study aimed to evaluate the efficacy of OCA in reducing CYC-induced thyroid toxicity and to investigate its underlying molecular mechanisms in rats. Rats were randomly assigned to five groups: control, OCA, CYC, OCA10 + CYC and OCA20 + CYC. Serum thyroid hormones (T3, T4), oxidative stress markers (malondialdehyde [MDA] and total antioxidant capacity [TAC]) and histopathological changes were assessed. Molecular analyses included the measurement of myeloid differentiation primary response gene 88 (MYD88), toll-like receptor 4 (TLR4), nuclear factor κB (NF-κB), interleukin (IL)-1β, IL-18, thioredoxin-interacting protein (TXNIP), NOD-like receptor protein 3 (NLRP3), apoptosis-associated speck-like protein (ASC), caspase-1, B-cell lymphoma-2 (BCL-2), BAX and caspase-3 expression. CYC administration resulted in significant thyroid damage, reflected by increased serum T3 and T4 levels, histopathological alterations, enhanced oxidative stress and the activation of inflammatory and apoptotic pathways. OCA pre-treatment significantly mitigated these changes as evidenced by lowered MDA, higher TAC and improved thyroid architecture. Mechanistically, OCA suppressed inflammation via downregulation of TLR4/MYD88/NF-κB signalling, inhibited pyroptosis through TXNIP/NLRP3/ASC/caspase-1 pathway blockade and reduced p53-mediated apoptosis by modulating BAX, BCL-2 and cleaved caspase-3 expression. This study provides the first evidence that OCA confers protection against CYC-induced thyroid injury by reducing oxidative stress, inflammation, pyroptosis and apoptosis.