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UNDERSTANDING MCT8 DEFICIENCY

MCT8 deficiency (Allan-Herndon-Dudley syndrome [AHDS]) is a rare genetic disorder caused by a change in the SLC16A2 gene. It makes it hard for the brain to get enough thyroid hormone while the rest of the body gets too much. This imbalance can lead to serious developmental impairment, feeding and growth problems, heart concerns, muscle weakness or stiffness, and overall health concerns.

Hearing that your child has MCT8 deficiency can be a devastating moment in your life as a parent or caregiver. Learning more about MCT8 deficiency can help you understand the challenges your child may face and advocate for the care they need.

WHAT IS MCT8 DEFICIENCY?

MCT8 deficiency is a rare genetic disorder that mostly affects boys. It happens because of changes in a gene called SLC16A2, which makes a protein called MCT8. This protein helps move thyroid hormones (especially one called T3) in and out of the cells, including the brain.

Thyroid hormones are very important for brain development and for controlling how the body uses energy. When MCT8 doesn’t work properly, the body and brain are affected in different ways.

MCT8 deficiency affects brain development and the function of other organs in the body, making lifelong support essential.

Brain icon.

In the brain:

T3 cannot get into brain cells, which causes low thyroid hormone levels in the brain. This leads to serious problems with brain development and learning.

Body icon.

In the body:

Too little T3 reaches the brain in MCT8 deficiency, but T3 levels in the blood stay high. This can lead to a condition called “thyrotoxicosis,” meaning some organs outside the brain get too much T3. Symptoms of thyrotoxicosis can include a fast or irregular heartbeat, ongoing loss of body weight, irritability, and sweating.

WHAT ARE SYMPTOMS OF MCT8 DEFICIENCY?

Baby icon.

Here is what to look for in the first months of life:

  • Trouble holding up the head
  • Delays in reaching developmental milestones
  • High T3 hormone levels, which may not cause obvious symptoms at first
  • Ongoing loss of body weight
  • Sleep problems
Checklist icon.

Over time, these signs and symptoms may appear or worsen:

Delays in movement and daily activities

  • Not able to sit without support
  • Not able to walk

Feeding challenges

  • Ongoing loss of body weight
  • Higher risk of food or liquid going into the airway (aspiration)
Brain-related and body-wide symptoms of MCT8 deficiency.

Problems with muscles

  • Muscles that feel weak
  • Muscles may become smaller
  • Ongoing muscle loss

Unusual movements

  • Moving less than expected
  • Muscle tightening or twisting movements
  • Sudden movement episodes may or may not be seizures

Heart-related issues

  • Heart beating too fast
  • Irregular heartbeats
  • High blood pressure

Sleep problems and increased fussiness or irritability

Frequent infections

Not gaining independence in everyday activities

Overall impact on daily life

  • These symptoms can reduce quality of life and may increase the risk of serious health problems over time
Brain-related and body-wide symptoms of MCT8 deficiency.

Parents and doctors should be on the lookout for typical signs and symptoms of MCT8 deficiency.

Are There Treatments for MCT8 Deficiency?

Currently, there are no approved treatment for MCT8 deficiency in the United States, creating a high unmet need. Care focuses on:

  • Managing symptoms
  • Preventing complications
  • Supporting comfort and quality of life

Medical teams work with families to manage nutrition, heart health, breathing, and muscle stiffness, often with help from several specialists. Supportive therapies like physical, occupational, and speech therapy help with comfort, positioning, and daily care. Most people need ongoing medical support, and care plans are tailored to each person’s needs.

While caring for someone with MCT8 deficiency can be challenging, working with a multidisciplinary healthcare team can help ensure the individual gets the care they need.

What Is It Like to Live With MCT8 Deficiency?

People with MCT8 deficiency can experience a wide range of symptom severity. Life expectancy also differs from person to person. However, even with high‑quality medical support, many face a reduced lifespan.

Those living with MCT8 deficiency typically need significant, ongoing assistance, either from devoted family caregivers or in a specialized care setting. While many children live into adulthood, about 1 in 3 do not, and those who do generally cannot walk independently or develop verbal communication skills.

As a result, they depend on caregivers for essential daily tasks, including eating, bathing, personal hygiene, dressing, mobility and transportation, and the administration of medications and other supportive therapies.

The complex impact of MCT8 deficiency
requires care from multiple specialists

Specialists who care for people with MCT8 deficiency.

Working closely with a team of healthcare professionals can make a real difference in the comfort and health of people with MCT8 deficiency.

Get Answers to 8 Frequently Asked Questions About MCT8 Deficiency.

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HELPFUL RESOURCES

EDUCATIONAL RESOURCES

Genetic and Rare Diseases Information Center (GARD – National Institutes of Health [NIH])

An NIH program that provides free access to reliable, easy-to-understand information about genetic and rare diseases.

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MedlinePlus Genetics (NIH/NLM)

An online health information resource for patients and their families and friends.

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National Organization for Rare Disorders (NORD) – Rare Disease Database

The NORD MCT8-Specific Thyroid Hormone Cell Transporter Deficiency Rare Disease Report is an authoritative source of information.

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SUPPORT RESOURCES

MCT8-AHDS Foundation

The foundation strives to improve the quality of life and potential of our children, support hundreds of families around the world, and more.

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Child Neurology Foundation – Support for Patients & Caregivers

The foundation focuses on the best quality of care and the highest quality-of-life for all members of the child neurology community.

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RareConnect – AHDS Online Community Forum

A safe, easy-to-use platform where rare disease patients, families, and patient organizations can develop online communities and conversations.

VISIT SITE

Patient Advocate Foundation

A national 501 (c)(3) non-profit organization that provides professional case management services to Americans with chronic, life-threatening, and debilitating illnesses.

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SEE HOW MCT8 DEFICIENCY IS DIAGNOSED

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