Rescue of Infants With MCT8 Deficiency

Inclusion Criteria * Genetic Confirmation: Male fetus or fetuses (including monozygotic twin pregnancies) must have a confirmed MCT8 gene mutation. * Family History: A previously born child or children with a severe, typical phenotype and an MCT8 gene mutation identical to that of the fetus. * Alternatively, the mother or a sister must have a relative with a known MCT8 defect. * Parental Decision: Parental refusal to terminate the pregnancy despite the diagnosis of MCT8 deficiency. * Compliance and Availability: Willingness of the parents to comply with all study procedures and ensure availability for the duration of the study. Exclusion Criteria: • Pregnancy-Related Factors: Dizygotic (non-identical) twin pregnancy (unless only one fetus is confirmed with the MCT8 mutation, and the unaffected fetus will not be treated). Parental decision to terminate the pregnancy. • Maternal Medical Conditions: Hyperthyroidism requiring treatment. Significant liver or kidney insufficiency. Congestive heart failure. Hyperemesis gravidarum unresponsive to treatment. * Significant cardiac conditions, including: * Atrial fibrillation or other arrhythmias. * Unstable angina. * Coronary heart disease. * Medications: Current use of sympathomimetic therapy. Anticoagulant therapy. Use of Cytochrome P450 2C9 (CYP2C9) inhibitors with a narrow therapeutic index. • Other Factors: Major illness or recent major surgery within four weeks of baseline visit 1, unrelated to MCT8 deficiency.