Deficiency of thyroid hormone leads to a generalized slowing of metabolic processes in the body. Signs and symptoms of hypothyroidism depend upon the underlying etiology, degree of deficiency, and the patient’s age. Although the diagnosis may be apparent in a child with the classic symptoms and physical exam findings, occasionally children and adolescents will present with a vague complaint of feeling unwell. In these cases, it is important to have a high index of suspicion for the disease.

Children may be affected by congenital or acquired hypothyroidism. Congenital forms are present at birth and are typically due to dysgenesis of the thyroid gland, though many other causes exist. Because thyroid hormone plays a critical role in early growth and central nervous system (CNS) development, unrecognized congenital hypothyroidism leads to irreversible mental retardation. Additional findings may include prolonged neonatal jaundice, enlarged fontanelles, macroglossia, hoarse cry, constipation, and poor weight gain. Newborn screening allows early diagnosis of congenital hypothyroidism; properly instituted treatment with thyroid hormone replacement allows for normal cognitive outcomes.

Acquired forms of hypothyroidism typically occur in older children, though there are rare reports of onset in infancy. Acquired hypothyroidism may be due to a primary disorder of the thyroid gland, or may be due to central deficiencies in either thyroid-stimulating hormone (TSH) or thyrotropin-releasing hormone (TRH). Primary hypothyroidism is associated with decreased levels of free thyroxine (T₄) and elevated TSH, the major factor controlling thyroid hormone synthesis and secretion. Normal or low TSH in a hypothyroid patient usually indicates a central cause.