This is a retrospective case report about a female patient with congenital hypothyroidism (CH), whose screening was missed at birth. Her mother noticed she was very calm, slept for long hours, including throughout the night, and had prolonged constipation. At 2.5 months, she was referred to a pediatrician after 20 days without stool excretion. Rectal touch by the pediatrician caused stool excretion in a large amount. Following a high TSH of 47 mIU/L, Levothyroxine 50 µg/day was prescribed and started at 3 months of age. TSH was reduced to 4.5 mIU/L soon after. At 3 years old, sonography showed a normal-sized thyroid gland in the correct position, but TSH increased to 13.8 mIU/L after stopping therapy for 18 days. Levothyroxine treatment was resumed and adjusted as needed. Thyroid function was followed and controlled with weekly 220 µg of Levothyroxine at the age of 20 years, and stopped after that, but some symptoms of subclinical hypothyroidism reappeared upon discontinuation, although endocrinologists believed that there is no need for treatment in TSH below 10 mIU/L. This case raises questions about the impact of early iodine exposure and familial hypothyroidism, too. Despite a 3-month delay in initiating therapy, the patient had normal IQ, EQ, and social intelligence according to the related tests.