Triac Trial II in MCT8 Deficiency Patients

Inclusion Criteria: * Signed and dated informed consent form from the parents or legal guardian. * Parents stated willingness to comply with all study procedures and availability for the duration of the study. * The participant should be aged between 0 and 30 months on the day of inclusion. * The participant should be male and have a pathogenic mutation in the MCT8 gene. Exclusion Criteria: * Previous treatment with tiratricol. * Previous treatment with LT4 and/or PTU and/or other anti-thyroid medication for a period longer than three months. Patients previously treated with LT4 for a shorter period than 3 months may be included in the study (baseline visit) six weeks (or longer) after last dose of LT4 if two consecutive analyses show stable TFT\*. Patients treated with PTU and/or other anti-thyroid medication for a shorter period than three months may be included in the study (baseline visit) six weeks (or longer) after last dose. * Major illness or recent major surgery (within four weeks of baseline visit 1) unrelated to MCT8 deficiency. * Known allergic reactions to components of the IMP. Patients with galactose intolerance, Lapp lactase deficiency or malabsorption of glucose or galactose (the IMP contains lactose). * Treatment with another investigational drug or participation in other interventional trial within three months prior to baseline visit 1.