Hi thanks for asking question. Intellectual function problem can be because of some genetic cause like down syndrome, klinefelter syndrome , congenital hypothyroidism , congenital iodine deficiency ,neurofibromatosis etc. If congenital causes suspected then search for other symptoms also like heart defect, genitalia examination , fingers proper or not , resiratory problems , skin nodules etc. Intellectual disturbance may be because of acquired causes like lack of iodine in food , malnutrition etc. For congenital causes confirmation genetic study like polymerase chain reaction or flourescent in situ hybridisation may needed. I hope my suggestion will help you. Wish you good health.