Congenital Hypothyroidism

Test

• Radioimmunoassay for T4 (thyroxine).  If T4 is in lowest 10%, TSH is measured on same specimen.  Approximately 10% of cases are detected only by a second screening at 2 to 6 weeks of age (due to transient TSH elevation at 24-48 hours of life).

Genetics

• Usually sporadic inheritance
• Some forms may be sex-linked; female-male ratio is 2-3:1
• 1:3,600 to 1:5,000 in US
• More prevalent in Hispanic and Native American populations

Pathology

• Embryogenic defects of the thyroid gland account for 85% of the cases of congenital hypothyroidism.  Inborn errors of metabolism of thyroid hormonogenesis account for 10-15%.  Embryogenic defects or inborn errors at the level of the pituitary or hypothalamus represent less than 4% of cases.  False positives may be caused by hereditarily decreased thyroid binding globulin.
• Fetal axis mature at midgestation.
• Some maternal transfer occurs -- cord levels are 1/3 of maternal in athyroid patients.
• Increased risk of congenital heart disease
• Preterm infants may have low T4 euthyroidism

Diagnosis

• Signs do not appear until the infant is at least several months of age, since there is placental transfer of thyroid hormone
• The most common neonatal signs are prolonged jaundice, constipation, and umbilical hernia
• Mental retardation -- 67% have IQs less than 85.  Deafness and other neurologic abnormalities possible.
• Physical findings may include poort growth, goiter, low metabolic rate, constipation, poor peripheral circulation, bradycardia, and myxedema.

Treatment

• Oral levothyroxine

Acknowledgment

Also see other Inborn Errors of Metabolism.
 

Genetics

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