The resistance to thyroid hormones appears to vary among different tissues and is believed to be due to a defect in the nuclear receptor. Finally, congenital hypothyroidism may be due to a variety of defects in the biosynthesis of thyroid hormone. The presence of a goiter is the hallmark of these defects, and the condition is termed goitrous hypothyroidism or goitrous cretinism. In most instances it is inherited in an autosomal recessive manner. The defects may involve iodide trapping, iodide organification, a coupling defect in which iodotyrosines are coupled to form diiodothyronines, deiodinase defects, and defect of thyroglobulin synthesis.
Congenital hypothyroidism is twice as common in girls as in boys.
The clinical signs and symptoms are usually absent or so nonspecific or subtle that the majority of infants look completely normal at birth. Since it is known that maternal thyroid hormones do not cross the placenta to any significant degree, it appears that thyroid hormones are not crucial for somatic growth during prenatal life, a fact that seems paradoxical when it is clear that they are essential for postnatal growth. Other clinical findings may be delayed stooling at birth and constipation, poor feeding owing to decreased appetite and lethargy, and prolonged jaundice secondary to immaturity of the hepatic glucuronyl transferase. Only a minority of patients will have the classic features of a puffy face, wide fontanelle and sutures, flattened nasal bridge with pseudohypertelorism, a large protruding tongue with an open mouth, a hoarse cry, a protuberant abdomen with an umbilical hernia, cold. mottled, or jaundiced skin, and sluggish reflexes. The most common dilemma facing the pediatrician is what to do with abnormal neonatal thyroid screening results.