MCT8-specific thyroid hormone cell transporter deficiency (THCT deficiency) is an inherited disorder that is characterized by severe mental retardation, an impaired ability to speak, diminished muscle tone (hypotonia), and/or movement abnormalities.

With the exception of poor muscle tone, most affected infants appear to develop normally during the first months of life. However, by about two months of age, affected infants may seem weak and have an inability to hold up the head. Due to hypotonia, severely reduced motor development, and other abnormalities, affected children very rarely develop any ability to walk and when they do, it is with shuffling gait. Associated features often include underdevelopment (hypoplasia) and wasting (atrophy) of muscle tissue; weakness and stiffness of the legs (spastic paraplegia) with exaggerated reflexes (hyperreflexia); relatively slow, involuntary, purposeless, commonly dyskinetic (abnormal movement) attacks. Writhing movements (athetoid movements); and/or other movement abnormalities are less common. Affected individuals may also have abnormalities of the skull and facial (craniofacial) region. THCT deficiency is inherited as an X-linked genetic disorder.