Most common cause of syndromic deafness is:

Pendred syndrome is the most common syndromic form of deafness, accounting for approximately 10% of cases. It presents with goiter and sensorineural hearing loss. Usually, the goiter is evident before pubey, but an adult onset has also been noted. Thyroid function is evenly divided, with 50% euthyroid patients and 50% hypothyroid patients. In most cases, the hearing loss is congenital, bilateral, moderate to profound, and sloping in the higher frequencies. An enlarged vestibular aqueduct is a consistent finding, and Mondini-type malformations have also been associated.