50 YEARS OF NEWBORN SCREENING FOR CONGENITAL HYPOTHYROIDISM: EVOLUTION OF INSIGHTS IN ETIOLOGY, DIAGNOSIS AND MANAGEMENT: Transient or permanent congenital hypothyroidism: from milestones to current and future perspectives.

Primary congenital hypothyroidism (CH) is the most common endocrinopathy of developmental age. In recent years, several studies from different countries have reported a significant increase in CH incidence detected by newborn screening programs, primarily 'mild' forms of CH with gland in situ (GIS). However, more than one-third of affected children with GIS present transient CH and recover endogenous thyroid function in early childhood, permitting the cessation of levothyroxine treatment by the end of the third year of life. Therefore, in CH patients with GIS, a clinical and biochemical reassessment is needed to determine whether the hypothyroidism is transient or permanent and to search for the underlying causes of the thyroid defect. Despite the presence of consensus guidelines for the management of CH in pediatric age, the screening strategy and management of the disease, especially at re-evaluation, differ significantly between centers and present some points of discussion. The following review summarizes the main pathophysiological mechanisms of transient and permanent forms of CH, also underlining the importance of new genetic tools in order to guarantee each patient the best diagnostic and therapeutic approach.