Clinical Research Directory

Inclusion Criteria: 1. Male participants diagnosed with a pathogenic mutation in the MCT8 gene, confirmed with a genetic test. 2. Serum total T3 concentration above the ULN of the age specific normal range: 1. at the time of diagnosis (or the closest sample taken prior to first ever treatment with tiratricol) for participants who are currently treated with tiratricol (if serum total T3 concentration is not available, free T3 results from standard of care samples may be used) 2. in the Screening Visit sample: i. For participants who have never received and/or are currently not receiving tiratricol. ii. For participants who stopped prohibited medications per exclusion criterion #6. 3. Participants will be aged 4 years or older at the time of randomization. Participants entering screening who are \<4 years of age but expected to be aged 4 years at randomization should be discussed with the medical monitor. 4. Signed and dated informed consent form from the parents or legal guardian. Exclusion Criteria: 1. Major illness or recent major surgery unrelated to MCT8 deficiency (in the principal investigator's judgement), defined as: * Conditions requiring repeated hospitalizations that are likely to confound ability to participate in the trial. * Major illness in the 3 months prior to the Screening Visit that is likely to confound the ability of the participant to participate fully within the trial and/or confound the assessment of serum total T3 and/or safety. * Major surgery within the 3 months prior to the Screening Visit or planned to take place during the study, including but not limited to major abdominal/thoracic/neurosurgical procedures. * Major/minor abdominal and/or maxillofacial surgery that may inhibit the administration and/or absorption of study drug. 2. Body weight \<10 kg at the Screening Visit. 3. Patients who are participating, or intend to participate, in other therapeutic and/or interventional clinical studies during the study period. 4. History of allergic reactions to components of tiratricol or any excipients in the investigational product (IP). 5. Participants with any contra-indication for treatment with tiratricol or any excipients in the IP. 6. Participants who have used other T3 analogues, levothyroxine, propylthiouracil, or other antithyroid medications within 6 weeks of screening. Randomization Criteria: In addition to the eligibility criteria, participants must meet further criteria at the time of randomization to enter the Randomized Treatment Period. 1. Confirmation that the "Stable Dose Criterion" has been met. 2. Absence of any new or exacerbated medical or surgical condition that fulfils Exclusion criterion #1. 3. Confirmation that participant is at least 4 years of age at the time of randomization.