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Congenital hypothyroidism is the deficiency of thyroid hormones in neonates and infants.
Cretinism is severe, untreated congenital hypothyroidism; it results in short stature (dwarfism) and intellectual disabilities.
Congenital hypothyroidism is 1 of the most common preventable causes of intellectual disabilities. Hence, early diagnosis and treatment are important to decrease morbidity from this condition.
Congenital hypothyroidism may occur as a result of an inborn error of the thyroid metabolism, iodine deficiency, or a structural defect in the thyroid gland.
Etiology and Pathophysiology
Anatomic defects of the thyroid gland
The thyroid gland develops from the 4th branchial pouch in the buccopharyngeal cavity during the 4th to 10th week of gestation. It later migrates toward the neck to rest at its anatomic position. An error in the migration or formation of the thyroid tissue (dysplasia, aplasia, or ectopy) may result in congenital hypothyroidism.
Iodine is required for the production of thyroid hormones triiodothyronine (T3) and thyroxine (T4) from the thyroid gland. A deficiency of iodine, therefore, results in decreased production of thyroid hormones.
Inborn errors of thyroid metabolism
Children with normal anatomic thyroid glands may still suffer from congenital hypothyroidism due to other inborn errors of thyroid metabolism. These inborn errors may include:
- Lack of iodide uptake
- Abnormalities in thyroid stimulating hormone (TSH) receptors
- Defects in thyroglobulin
- Deiodinase defect
- Defects in thyroid peroxidase
In the United States, it is estimated that the annual incidence of congenital hypothyroidism is approximately 1 in 4,000 births. Data from a number of other countries indicate a similar incidence, at approximately 1 in 3,000–4,000 births. However, the incidence is higher in some Middle Eastern countries, at approximately 1 in 1,400–2,000 births.
Racial and gender disparities have also been noted; for example, the incidence of congenital hypothyroidism is higher among Hispanic females. It is also higher in girls than boys, with a female-to-male ratio of 2:1. Twin births appear to have a 12-times-higher risk of developing the condition.
Newborn infants with congenital hypothyroidism are usually born at normal gestational term or even post-term and may be initially asymptomatic, as maternal hormones pass on to the fetus through the placenta.
Birth length and weight are usually within the normal range, although head circumference may be increased. The knee epiphyses often lack calcification. Common clinical features include:
- Poor growth
- Small stature
- Poor feeding resulting in poor weight gain
- Decreased activity
- Hoarse cry
Affected babies often sleep excessively and rarely cry, and are thus labeled as “good babies.”
On physical examination, the following signs may be present:
- Macroglossia and coarse facial features
- Large anterior fontanelle
- Umbilical hernia
- Slow development
- Dry skin
Screening newborns for congenital hypothyroidism
In the United States and many other developed countries, screening for congenital hypothyroidism is performed during the 1st week of life, which identifies those children suffering from the condition during this asymptomatic stage. A careful history and physical examination should be performed and testing should be repeated in order to confirm the diagnosis.
Early diagnosis and treatment of congenital hypothyroidism are important to prevent long-term mental disability from this condition, as thyroid hormones are crucial for normal growth and maturation of the brain.
- Thyroxine-binding globulin deficiency
- Pediatric hypopituitarism
- Beckwith-Wiedemann syndrome
- Elevated levels of TSH
- Decreased levels of total and free thyroid hormone (T4)
- Neonatal and maternal antithyroid antibodies may be found
- Technetium-99m or iodine-123 may be useful in determining the cause of hypothyroidism.
- Ectopic thyroid may be demonstrated by thyroid scans.
- Ultrasonography may be used as an alternative to thyroid scans.
Early diagnosis of the condition is very important for management. Medical treatment involves thyroid hormone replacement (oral levothyroxine is used).
In endemic areas where iodine deficiency is common, dietary iodide supplementation is advised and may help prevent this condition.
Prognosis is excellent if treatment is timely. Most children achieve normal physical and mental growth status. Regular follow-ups and blood thyroid hormone measurements are needed for optimum management.