Withdrawal of Tiratricol Treatment in Males with Monocarboxylate Transporter 8 Deficiency (MCT8 Deficiency)

Inclusion Criteria:

1. Male participants diagnosed with a pathogenic mutation in the MCT8 gene, confirmedwith a genetic test.

2. Serum total T3 concentration above the ULN of the age specific normal range:

3. at the time of diagnosis (or the closest sample taken prior to first evertreatment with tiratricol) for participants who are currently treated withtiratricol

4. in the Screening Visit sample, or most recent standard of care sample prior toscreening, for participants who have never received and/or currently notreceiving tiratricol.

5. 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 4years at randomization should be discussed with the medical monitor.

6. 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 principalinvestigator's judgement), defined as:

• Conditions requiring repeated hospitalizations that are likely to confoundability to participate in the trial.

• Major illness in the 3 months prior to the screening visit that is likely toconfound the ability of the participant to participate fully within the trialand/or confound the assessment of serum total T3 and/or safety.

• Major surgery within the 3 months prior to the screening visit or planned totake place during the study, including but not limited to majorabdominal/thoracic/neurosurgical procedures.

• Major/minor abdominal and/or maxillofacial surgery that may inhibit theadministration 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 therapeuticand/or interventional clinical studies during the study period.

4. History of allergic reactions to components of tiratricol or any excipients in theinvestigational product (IP).

5. Participants with any contra-indication for treatment with tiratricol or anyexcipients in the IP.

6. Participants using other T3 analogues, levothyroxine, or propylthiouracil.

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 fulfilsExclusion criterion #1.

3. Confirmation that participant is at least 4 years of age at the time ofrandomization.