Hypothyroidism is thyroid hormone deficiency. Symptoms in infants include poor feeding and growth failure; symptoms in older children and adolescents are similar to those of adults but also include growth failure, delayed puberty, or both. Diagnosis is by thyroid function testing (eg, serum thyroxine, thyroid-stimulating hormone). Treatment is thyroid hormone replacement.

Etiology

Hypothyroidism in infants and young children may be congenital or acquired.

Congenital hypothyroidism occurs in about 1/4000 live births. Most congenital cases are sporadic, but about 10 to 20% are inherited. The most frequent cause of congenital hypothyroidism is dysgenesis, either absence (agenesis) or underdevelopment (hypoplasia) of the thyroid gland. About 10% of congenital hypothyroidism results from dyshormonogenesis (abnormal thyroid hormone production), of which there are 4 types (see Endocrine Disorders in Children: Congenital Goiter). Rarely in the US but commonly in certain developing countries, hypothyroidism results from maternal iodine deficiency. Rarely, transplacental transfer of antibodies, goitrogens (eg, amiodaroneSome Trade Names
CORDARONE
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), or antithyroid drugs (eg, propylthiouracilSome Trade Names
No US trade name
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, methimazoleSome Trade Names
TAPAZOLE
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) causes transient hypothyroidism.

Acquired hypothyroidism is typically caused by autoimmune thyroiditis (Hashimoto's thyroiditis) and occurs during later childhood and adolescence.

Symptoms and Signs

Symptoms and signs in infants and young children differ from those in older children and adults. If iodine deficiency occurs very early during pregnancy, infants may present with endemic cretinism (a syndrome involving deaf-mutism), intellectual disability, and spasticity. Most other hypothyroid infants initially have few if any symptoms or signs. Symptoms that do occur may be subtle or develop slowly because some maternal thyroid hormone crosses the placenta. However, after the maternal thyroid hormone is metabolized, if the underlying cause of hypothyroidism persists and hypothyroidism remains undiagnosed or untreated, it usually slows CNS development moderately to severely and may be accompanied by low muscle tone, prolonged hyperbilirubinemia, umbilical hernia, respiratory distress, macroglossia, large fontanelles, poor feeding, and hoarse crying. Rarely, delayed diagnosis and treatment of severe hypothyroidism lead to intellectual disability and short stature.

Some symptoms and signs in older children and adolescents are similar to those of adults (eg, weight gain; constipation; coarse, dry hair; sallow, cool, or mottled coarse skin—see Thyroid Disorders: Symptoms and Signs). Signs specific to children are growth retardation, delayed skeletal maturation, and usually delayed puberty.

Diagnosis

• Routine neonatal screening
• Thyroid function tests

Routine neonatal screening detects hypothyroidism before clinical signs are evident. If screening is positive, confirmation is necessary with thyroid function tests, including measurement of serum thyroxine (T₄), free T₄, and thyroid-stimulating hormone (TSH). These tests are also done in older children and adolescents in whom hypothyroidism is suspected.

Severe congenital hypothyroidism, even when treated promptly, may still cause subtle developmental problems and sensorineural hearing loss. Hearing loss may be so mild that initial screening misses it, although it may still interfere with language acquisition. Retesting after infancy is advised to detect subtle hearing loss.

Treatment

• Thyroid hormone replacement

Most cases of congenital hypothyroidism require lifelong thyroid hormone replacement. Treatment with l-thyroxine 10 to 15 μg/kg po once/day must be started immediately and be closely monitored. This dosage is intended to rapidly normalize serum T₄ and should then be adjusted to maintain the serum T₄ level between 10 and 15 μg/dL during infancy. After age 1 yr, the usual dosage is 5 to 6 μg/kg po once/day, titrated to maintain serum T₄ and TSH levels within the normal range for age. This dosing regimen is also used for acquired hypothyroidism in children and adolescents. In later childhood or adolescence, the starting dosage may be calculated as 100 μg /m² po once/day. In most treated infants, motor and intellectual development is normal. Thyroxine-binding globulin deficiency, detected by screening that relies primarily on T₄ measurement, does not require treatment because affected infants are euthyroid.

Last full review/revision May 2009 by Nicholas Jospe, MD

Content last modified February 2012