Triple X syndrome (47,XXX) is a numerical chromosomal alteration that affects 1/1,000 women, in which the woman is born with an extra X chromosome. Some oral changes have been reported in the literature, as hypodontia, influence on deposition of crown enamel and discrepancies in cephalometric measurements. Other systemic complications may lead to oral abnormalities similar to those seen in triple X patients, such as congenital hypothyroidism (CH). This paper reports a triple X syndrome case associated with CH later treated. Besides delay in cognitive and intellectual development, the patient had changes in teeth development and in cephalometric measurements with deficiencies in the maxilla and mandible. This is the first report of a triple X syndrome associated with CH. Both conditions may result in changes in dentofacial development.