Endemic cretinism has been classified into neurological and myxedematous types. Profound mental deficiency, deaf-mutism and cerebral diplegia are predominantly found in the former. The latter have been described as less mentally retarded but with severe growth retardation and myxedematous features. The pathogenesis of different clinical types of endemic cretinism is still unclear. Recently, a unifying hypothesis suggested that iodine deficiency, severe enough to cause maternal and fetal hypothyroxinemia, results in neurological defects in all cretins. We conducted the present study in northern Thailand to determine the validity of this hypothesis in another geographical area. The study consisted of a multidisciplinary survey on 112 endemic cretins aged 2-66 years in Nan. They were categorized clinically into three types of endemic cretins, neurological (n = 57), myxedematous (n = 19) and mixed form (n = 36). The subjects were generally short and the majority had severe mental retardation (mean intellectual quotient (I.Q.) 30.8 +/- 8.8), psychomotor defect and profound sensorineural hearing loss. The I.Q. score and proportion of cretins with sensorineural hearing loss and psychomotor defect were similar among the three types of cretins. The most frequent neurological abnormalities were spasticity, hyper-reflexia, the presence of primitive reflexes and gait disturbance. These abnormalities were distributed equally among the three types of endemic cretins. Delayed skeletal maturation and abnormal epiphysis were also present in all types of cretins. However, myxedematous cretins were shorter (P < 0.01), having more myxedematous features (P < 0.05 to P < 0.001) and less sexual maturation (P < 0.05). Thyroid volume was lower in cretins with hypothyroidism (P < 0.01). In conclusion, our findings support the hypothesis that neurological features are present in all types of cretins, and are the consequence of maternal and fetal hypothyroxinemia due to severe iodine deficiency. The clinical manifestations of the cretins were subsequently modified by the length and severity of postnatal iodine deficiency and hypothyroidism.