Allan-Herndon- Dudley (AHDS) is caused by a mutation of the MCT8 gene which is responsible for the transport of Thyroid Hormones to the brain. It is an X-link inherited mutation that only affects male patients. Because of the mutation, the blood brain barrier does not recognize the carrier and it doesn’t allow its passage into the brain. As a result, the brain doesn’t develop properly. Lack of thyroid hormones in the brain is the main cause for mental delay. So, the hormones are stuck in the body creating toxicity in the blood and putting pressure on vital organs.