sentences:  Consequently, thyroid hormones are unable to enter the nervous system, which depends on thyroid signaling for proper function and development.It is estimated that 80–99% of people with Allan-Herndon-Dudley syndrome will have biparietal narrowing (narrowing of skull), ataxia, abnormalities of the neck, and both absent speech development and aphasia.Weak muscle tone (hypotonia) and underdevelopment of many muscles (muscle hypoplasia) are common in children with Allan-Herndon-Dudley syndrome.