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Hypothyroidism in Infants and Children

by Andrew Calabria, MD

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

Congenital hypothyroidism occurs in about 1/2000 to 1/4000 live births. Most congenital cases are sporadic, but about 10 to 20% are inherited. The causes usually involve

  • Dysgenesis of the gland (85% of cases)

  • Dyshormonogenesis (abnormal thyroid hormone production, 10 to 15% of cases)

Dysgenesis may involve ectopy (two thirds of cases), absence (agenesis), or underdevelopment (hypoplasia) of the thyroid gland.

Dyshormonogenesis has multiple types, which can result from a defect in any of the steps of thyroid hormone biosynthesis (see 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, amiodarone), or antithyroid drugs (eg, propylthiouracil, methimazole) causes transient hypothyroidism. Another rare cause is central hypothyroidism, which is caused by structural anomalies in pituitary development; patients usually also have other pituitary hormone deficiencies.

Acquired hypothyroidism

Acquired hypothyroidism is typically caused by autoimmune thyroiditis (Hashimoto thyroiditis—see Hashimoto 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 severe growth failure, coarse facial features, intellectual disability, and spasticity. Most other hypothyroid infants initially have few if any symptoms or signs and are detected only through newborn screening.

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, sensorineural hearing loss, 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; fatigue; constipation; coarse, dry hair; sallow, cool, or mottled coarse skin—see Subclinical Hyperthyroidism : Symptoms and Signs). Signs specific to children are growth retardation, delayed skeletal maturation, and usually delayed puberty.

Diagnosis

  • Routine newborn screening

  • Thyroid function tests

  • Sometimes thyroid ultrasonography or radionuclide scan

Routine newborn screening detects hypothyroidism before clinical signs are evident. If screening is positive, confirmation is necessary with thyroid function tests, including measurement of serum thyroxine (T 4 ), free T 4 , 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, but it may still interfere with language acquisition. Retesting after infancy is advised to detect subtle hearing loss.

When congenital hypothyroidism is diagnosed, radionuclide scanning (either 99m Tc pertechnetate or 123 I) or ultrasonography can be done to evaluate the size and location of the thyroid gland and thus help distinguish a structural abnormality (ie, thyroid dysgenesis) from dyshormonogenesis and transient abnormalities.

In older children and adolescents with suspected hypothyroidism (elevated TSH, low T 4 , or free T 4 ), thyroid antibody titers (to thyroid peroxidase and thyroglobulin) should be measured to evaluate for autoimmune thyroiditis. Thyroid ultrasonography is not necessary to establish the diagnosis of autoimmune thyroiditis and should be restricted to children with thyroid gland asymmetry or palpable thyroid nodules.

Treatment

  • Thyroid hormone replacement

In most treated infants, motor and intellectual development is normal.

When to treat

Most cases of congenital hypothyroidism require lifelong thyroid hormone replacement. However, if the initial TSH level is < 40 mU/L, an organic basis is not established, and the disease is thought to be transient, clinicians may try stopping therapy after age 3 yr, at which time the trial poses no danger to the developing CNS. If the TSH rises once therapy is stopped and the free T 4 or T 4 is low, permanent congenital hypothyroidism is confirmed and treatment should be restarted. Thyroxine-binding globulin deficiency, detected by screening that relies primarily on total serum T 4 measurement, does not require treatment because affected infants have normal free T 4 and TSH levels and are thus euthyroid.

Older children who have only slight elevations in TSH (< 10 mU/L) and normal T 4 or free T 4 levels are considered to have subclinical hypothyroidism whether they have thyroid autoantibodies or not. Such children do not need thyroid replacement unless they develop symptoms of hypothyroidism or goiter or their levels of TSH increase.

Treatment regimens

In congenital hypothyroidism, treatment with l -thyroxine 10 to 15 mcg/kg po once/day must be started immediately and be closely monitored. This dosage is intended to rapidly (within 2 wk) bring the serum T 4 level into the upper half of the normal range for age (between 10 μg/dL and 15 μg/dL) and promptly (within 4 wk) reduce the TSH.

After age 1 yr, the usual starting dosage of l -thyroxine is 4 to 6 mcg/kg po once/day, titrated to maintain serum T 4 and TSH levels within the normal range for age. This dosing regimen is also used for acquired hypothyroidism in children. In later childhood or adolescence, the starting dosage is 2 to 3 mcg/kg po once/day.

Thyroid replacement should be given only as a tablet, which can be crushed and made into paste for infants; it should not be given simultaneously with soy formula, or iron or Ca supplements, all of which can decrease thyroid hormone absorption.

Monitoring

Children are monitored more frequently during the first few years of life:

  • Every 1 to 2 mo during the 1st 6 mo

  • Every 3 to 4 mo between age 6 mo and 3 yr

  • Every 6 to 12 mo from age 3 yr to the end of growth

Older children can be monitored more frequently if there are concerns about adherence. After a dose adjustment in older children, TSH and T 4 levels are measured in 6 to 8 wk.

Key Points

  • Hypothyroidism in infants is usually congenital; acquired causes become more common with age.

  • Most congenital causes involve dysgenesis of the gland, but genetic disorders affecting thyroid hormone synthesis may occur.

  • Most hypothyroid infants are detected through routine newborn screening.

  • Confirm diagnosis with serum thyroxine (T 4 ), free T 4 , and thyroid-stimulating hormone (TSH); if confirmed, do imaging tests to detect structural thyroid disorders.

  • Treat with l -thyroxine, adjusting the dose to maintain T 4 and TSH levels within the normal range for age.

Drugs Mentioned In This Article

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  • TAPAZOLE
  • CORDARONE

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