Hypothyroidism is the condition of thyroid hormone deficiency. It can be primary or acquired. Primary hypothyroidism can be congenital or late onset. The symptoms of congenital hypothyroidism may go unnoticed in newborns if undiagnosed. Untreated, hypothyroidism can lead to poor mental and intellectual development in children. Hypothyroidism’s clinical manifestations are often subtle or not present at birth. Common symptoms include decreased activity and increased sleep, feeding difficulty, and constipation. On examination, common signs include myxoedematous facies, large fontanels, macroglossia, a distended abdomen with umbilical hernia, and hypotonia. Levothyroxine is the treatment of choice. In general, the prognosis is excellent when this condition is detected by screening and started on treatment early.