Monocarboxylate Transporter 8 (MCT8) deficiency (that is also known as Allan-Herndon-Dudley syndrome) is a rare X-linked inherited disorder of brain development that causes severe intellectual disability and problems with movement. This condition, which occurs almost exclusively in males, disrupts development from before birth.

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.

MCT8 mutation carrier mother with previously affected MCT8 deficient male will be screened for male fetal DNA as soon as she aware she is pregnant (week 4-7).