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SEE THE DIAGNOSIS PATHWAY

MCT8 deficiency (Allan-Herndon-Dudley syndrome [AHDS]) is a rare condition. Babies often show early signs such as weak muscle tone, slow growth, and problems with brain development and learning. Although signs appear in the first months of life, the age of diagnosis varies greatly.

Thyroid hormone testing is an important clue. Children with MCT8 deficiency usually have high T3, low or low-normal free T4, and normal or mildly elevated thyroid-stimulating hormone (TSH). Recognizing this pattern can help doctors identify the condition earlier and begin care sooner.

What Tests Are Done for Diagnosis?

Boys who show developmental impairment, low muscle tone, fast resting heart rate, and ongoing loss of body weight should have thyroid hormone testing that includes measuring levels of the hormones T3, T4, and TSH.

The testing result associated with MCT8 deficiency is:

  • High T3
  • Low to low-normal T4
  • Normal or slightly elevated TSH

To confirm the diagnosis, testing should also include a genetic test to look for mutations in the SLC16A2 gene.

Mother with doctor holding baby's hand.

Doctors diagnose MCT8 deficiency using thyroid hormone testing and genetic testing. Its important that the hormone testing checks T3 levels.

Which Doctors Diagnose and Manage MCT8 Deficiency?

Families meet with doctors who understand MCT8 deficiency, such as a pediatric neurologist or pediatric endocrinologist, as well as a geneticist and a genetic counselor. These specialists can explain the genetic testing process and help the family understand what it means if a genetic change is found, and how it may be passed down through the family.

MCT8 deficiency can cause serious challenges like ongoing loss of body weight, movement problems, and heart issues. Because these problems affect different parts of the body, care can be complicated. That’s why having a team of specialists working together is so important.

People in multidisciplinary care team.

Here are a 3 points to know about a multidisciplinary care team:

  1. In addition to care from endocrinologists and pediatricians, other experts can help with everyday needs like feeding and mobility
  2. When specialists share their knowledge and work as a team, they can help to address serious problems like recurring infections, especially pneumonia, ongoing loss of body weight, and heart complications
  3. A team approach can help address the many challenges children with MCT8 deficiency can face

Working with a multidisciplinary care team can help your child get the care they need.

WHY IS DIAGNOSIS SOMETIMES DELAYED?

MCT8 deficiency shows symptoms early in life, but diagnosis is often delayed.

  • Diagnosis can take about 14 months from first symptoms to a confirmed diagnosis.
  • For children born after 2017, this delay has improved to about 3 months, but late diagnoses still happen.

Diagnosis is further complicated because MCT8 deficiency is rare and unfamiliar to many clinicians, and standard thyroid testing often does not include T3 levels, which are critical for identifying the disorder.

When diagnosis is delayed, children may miss out on important care they need. This may lead to serious problems, like ongoing loss of body weight, recurrent respiratory infections, and heart issues.

Baby icon.
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Early and accurate diagnosis of MCT8 deficiency is essential to help ensure timely monitoring, supportive care and informed decisions for the families.

What Other Conditions Could MCT8 Deficiency Be Mistaken For?

Some medical conditions can have symptoms that look similar to those seen in MCT8 deficiency. Knowing about these conditions can help doctors consider and rule out other possible diagnoses.
  • Cerebral palsy: A condition that starts in early childhood and affects movement and muscle control because of changes in brain development. Learn more
  • Pelizaeus-Merzbacher (like) disease: A rare genetic disorder that affects the brain’s white matter, causing problems with movement, coordination, and development. Learn more
  • MECP2 duplication syndrome: A genetic condition where extra copies of the MECP2 gene cause impaired development, low muscle tone, and other neurological symptoms. Learn more
  • Mitochondrial disorders: A group of disorders where the cells’ energy factories (mitochondria) don’t work properly, leading to problems in organs like the brain, muscles, and heart. Learn more
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A HELPFUL RESOURCE ABOUT MCT8 DEFICIENCY DIAGNOSIS

National Organization for Rare Disorders (NORD)

—MCT8-Specific Thyroid Hormone Cell Transporter Deficiency Diagnosis

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