Rates of elderly patients with inflammatory bowel diseases (IBDs) are increasing, and biomarkers are needed to optimise their therapies. Serum triiodothyronine-to-thyroxine (T3/T4) ratio has been correlated with geriatric patient frailty. To assess the suitability of T3/T4 ratio as a response marker to biologics in elderly patients with IBD. Patients with IBD over 60 years old were enrolled, when starting biological therapy. Therapeutic outcome was assessed after 54 weeks of treatment as mucosal healing (Mayo endoscopic score < 2 for ulcerative colitis; ulcer disappearance for Crohn's disease) and clinical remission (Partial Mayo Score < 2 for ulcerative colitis; Harvey-Bradshaw Index < 5 for Crohn's disease). T3/T4 ratio was evaluated at baseline, and its association with therapeutic outcomes was tested by multivariable logistic regression and receiver operating characteristic (ROC). We enrolled 80 patients; 44 achieved clinical remission and 36 mucosal healing. Baseline T3/T4 ratio was higher in patients with mucosal healing, as compared with those without mucosal healing (P < 0.0001), regardless of the disease type or biological drug (OR 6.4 [2.9-14.3] for each T3/T4 unit increase, P < 0.0001). A cut point of 3.3 was identified as the optimal threshold of baseline T3/T4 ratio for predicting mucosal healing, providing 78% sensitivity and 89% specificity (area under the ROC curve 0.88 [0.79-0.94]; positive and negative likelihood ratios 6.8 [2.9-15.9] and 0.3 [0.1-0.5] respectively). T3/T4 ratio seems a reliable tool for predicting therapeutic outcome of biological therapy in elderly patients with IBD. If validated, the assessment of this parameter before starting biological treatment might be suggested.