Original
Allan-Herndon-Dudley syndrome, also referred to as AHDS or MCT8 is caused by a mutation of the SLC16A2 gene. This gene contains the instructions to create a certain protein that has the roll of transporting thyroid hormones into the brain cells. Due to the mutation, the blood brain barrier does not allow passage into the brain for the protein that is transporting the hormone.
Thyroid hormones are essential for the brain development of children, particularly in the first years of life. The lack of thyroid hormones in the brain leads to severe mental delay.
Here is a presentation explaining in simple terms what happens in AHDS.
https://prezi.com/cfvcwd6mwylb/children-with-allan-herndon-dudley-syndrome/