Understanding anti‐TNF treatment failure: does serum triiodothyronine‐to‐thyroxine (T3/T4) ratio predict therapeutic outcome to anti‐TNF therapies in biologic‐naïve patients with active luminal Crohn's disease?

BACKGROUND: During illness, adaptations of the hypothalamic–pituitary‐thyroid axis reduce energy expenditure, protein catabolism and modulate immune responses to promote survival. Lower serum free triiodothyronine‐to‐thyroxine (fT3/fT4) ratio has been linked to non‐response to treatment in a range of diseases, including in biologic‐treated patients with inflammatory bowel disease. AIM: To assess whether baseline serum fT3/fT4 ratio predicted primary non‐response (PNR) and non‐remission to infliximab and adalimumab in patients with Crohn's disease METHODS: Thyroid function tests were undertaken in stored serum from biologic‐naïve adult patients with active luminal Crohn's disease immediately prior to treatment with infliximab (427 originator; 122 biosimilar) or adalimumab (448) in the Personalised Anti‐TNF Therapy in Crohn's Disease study (PANTS). RESULTS: Baseline median [IQR] fT3/fT4 ratios were lower in women than men (0.30 [0.27–0.34] vs 0.32 [0.28–0.36], p < 0.001), in patients with more severe inflammatory disease, and in patients receiving corticosteroids (0.28 [0.25–0.33] vs. 0.32 [0.29–0.36], p < 0.001). Multivariable logistic regression analysis demonstrated that fT3/fT4 ratio was independently associated with PNR at week 14 (odds ratio [OR] 0.51, 95% confidence interval [CI] 0.31–0.85, p = 0.009), but not non‐remission or changes in faecal calprotectin concentrations at week 54. The optimal threshold to determine PNR was 0.31 (area under the curve 0.57 [95% CI 0.54–0.61], sensitivity 0.62 [95% CI 0.41–0.74], and specificity 0.53 [95% CI 0.42–0.73]). CONCLUSIONS: Lower baseline serum fT3/fT4 ratio was associated with female sex, corticosteroid use and disease activity. It predicted PNR to anti‐TNF treatment at week 14, but not non‐remission at week 54.