Growth is one of the most important measures of health in pediatrics. The American Academy of Pediatrics recommends that a child’s height and weight be measured at each well-child visit. Short stature is defined as being more than two standard deviations below the mean sex-matched height for age. The two most common causes of short stature are genetic short stature, in which the child is growing within the parental target range, and constitutional growth delay, in which the bone age is delayed. Children with endocrine disorders are usually overweight-for-height, in contrast to children with nutritional deficiencies who are underweight-for height. Hypothyroidism is a cause of growth failure in children. Hypothyroidism in infancy and early childhood is also one of the most common preventable causes of mental retardation. Pediatricians must include hypothyroidism in their differential diagnosis of patients with short stature and developmental delay.

Hypothyroidism in children with developmental delay may be either congenital or acquired. Congenital hypothyroidism (CH), an irreversible disorder that affects 1 in 4,000 live births, usually results from an abnormality in thyroid gland development (dysgenesis or agenesis) or an inborn error in thyroid hormonogenesis. Less commonly, CH may result from a pituitary or hypothalamic abnormality. Most neonates with CH have a normal appearance and no detectable physical signs. Timely newborn screening, and thyroid therapy started within 2 weeks of delivery, can ensure normal cognitive development. Persistent hypothyroidism, as defined by delayed return of serum thyroxine (T₄) levels to normal after initiation of treatment, may lead to adverse cognitive outcomes; therefore, a history of treated CH may not exclude hypothyroidism as a cause of short stature and developmental delay.

Worldwide, the most common cause of hypothyroidism is iodine deficiency. Acquired hypothyroidism in children in the United States usually results from chronic autoimmune thyroiditis (AKA Hashimoto thyroiditis), with goiter being a more common presentation of this disease than thyroid
atrophy. Children with some chromosomal abnormalities, such as trisomy 21, Turner syndrome, and Klinefelter syndrome, as well as children with Type 1 diabetes mellitus have an increased risk for chronic autoimmune thyroiditis. Another cause of hypothyroidism is thyroid injury, which may result from tumors of the head and neck region, craniospinal radiation, and radioiodine treatment for Graves’ hyperthyroidism. Hypothalamic or pituitary disease causes central hypothyroidism, and should be suspected when the thyroid-stimulating hormone (TSH) is low.

Developmental delay from congenital hypothyroidism may occur even in infants who have normal T₄ and TSH newborn screening results. CH may manifest or be acquired after newborn screening. There are some proponents of retesting after 4 weeks in all neonates with low and very low birth weight; therefore, clinicians should not hesitate to repeat thyroid screening.