Abstract Background: MCT8 deficiency is a severe disorder caused by mutations in the thyroid hormone transporter MCT8. MCT8 deficiency is characterized by severe intellectual and motor disability and high serum T3 concentrations that result in thyrotoxic symptoms in peripheral tissues. This predisposes to substantial morbidity and mortality. Preclinical studies showed that the T3 analogue Triac can bypass defective MCT8 at the cellular level. Recently, we reported the results of an international multicenter trial, in which biochemical and clinical outcomes improved in patients with MCT8 deficiency who were treated with Triac for 12 months (1). However, long-term follow-up data of patients with MCT8 deficiency treated with Triac are lacking, particularly in young children. Therefore, we aimed to investigate the long-term efficacy of Triac therapy in a worldwide cohort of patients with MCT8 deficiency. Methods: We investigated the efficacy of oral Triac treatment in pediatric (n=78) and adult (n=5) patients with MCT8 deficiency in 20 countries. Triac dose was titrated according a predefined dose-escalation scheme aiming to normalize serum T3 concentrations (target 1.4-2.5 nmol/L). Thyroid function tests and biochemical markers of thyroid hormone action in peripheral tissues (SHBG, creatine kinase, creatinine) were measured at baseline and during control visits. Findings: In total, 83 patients with a median baseline age of 5 years (range 6 months – 66 years) were treated, including 24 patients aged 0-2.5 years and 17 patients aged 2.5-5 years. They were treated with Triac during 144 patient years, of whom the follow-up time was >5 years in 9 patients and 2-5 years in 22 patients. Mean dose was 45 µg/kg/day (range 11-107 µg/kg/day). Once a stable dose was achieved, no further dose adjustments were needed. Mean serum T3 concentrations decreased from 5.02 to 1.94 nmol/L (normal 1.4 – 2.5 nmol/L). SHBG concentrations improved from 238 to 204 nmol/L (normal 40-140 nmol/L). Mean creatine kinase and creatinine concentrations improved from 113 to 140 U/L (normal <230 U/L) and from 32 to 38 µmol/L (normal 31-68 μmol/L), respectively. No drug-related severe adverse events were reported. Interpretation: Triac is a safe treatment that results in sustainable improvements of the severe thyrotoxic state in pediatric and adult patients with MCT8 deficiency. References: 1. Groeneweg S, Peeters RP, Moran C, Stoupa A, Auriol F, Tonduti D, et al. Effectiveness and safety of the tri-iodothyronine analogue Triac in children and adults with MCT8 deficiency: an international, single-arm, open-label, phase 2 trial. Lancet Diabetes Endocrinol. 2019;7(9):695-706.