We report one case of congenital hypothyroidism with thyroid agenesis wrongly considered as a transient form of hypothyroidism. The patient came to our attention at 9 months of age, showing an important delay in neuromotor development. Transient hypothyroidism is a situation of brief hormonal deficit with spontaneous resolution in the first weeks of life. We consider it very important to differentiate this transient condition from a permanent form of thyroid dysgenesis.