Thyroid functions in the fetus and newborn carry importance in terms of the baby's health and development of the central nervous system. Maternaliodine deficiency, exposure to iodine, thyroid diseases (Hashimoto thyroiditis, Graves') and drugs used by the mother affect thyroid functions in the fetus. Reflections of these effects are observed immediately after delivery. Investigation of the mother in terms of thyroid diseases during pregnancy, recognition and appropriate assessment of the required conditions, screening of all newborns in the first days of life in terms of congenital hypothyroidism, timely and appropriate evaluation of the screening results, early diagnosis and appropriate treatment of cases of congenital hypothyroidism, assessment and management of cases of transient thyroid hormone disorders and close monitoring of the thyroid functions and development of patients in whom treatment has been initiated with a diagnosis of hypothyroidism are crucial in terms of developmental outcomes of the babies who have thyroid function disorders or hypothyroidism. This guideline was written with the objective of guiding pediatricians, neonatologists and pediatric endocrinologists in the issue of assessment, diagnosis and management of thyroid function disorders and thyroid diseases concerning the fetus and baby during gestation and neonatal period.