Congenital Hypothyroidism
Congenital hypothyroidism is a condition that
affects infants from birth (congenital) and results
from a partial or complete loss of thyroid function
(hypothyroidism). The thyroid gland is a butterfly-shaped
tissue in the lower neck. It makes iodine-containing
hormones that play an important role in regulating growth,
brain development, and the rate of chemical reactions
in the body (metabolism).
Congenital hypothyroidism occurs when the thyroid gland
fails to develop or function properly. In 80 to 85 percent
of cases, the thyroid gland is absent, abnormally located,
or severely reduced in size (hypoplastic). In the remaining
cases, a normal-sized or enlarged thyroid gland is present,
but production of thyroid hormones is decreased or absent.
If untreated, congenital hypothyroidism can lead to
mental retardation and abnormal growth. In the United
States and many other countries, all newborns are tested
for congenital hypothyroidism. If treatment begins in
the first month after birth, infants usually develop
normally.
Genetic Prevalence
Studies of populations from North America, Europe, Japan,
and Australia, indicate that congenital hypothyroidism
affects 1 in 3,000 to 4,000 newborns. For reasons that
remain unclear, congenital hypothyroidism affects more
than twice as many females as males.
Mutations in the DUOX2, PAX8, SLC5A5, TG, TPO, TSHB,
and TSHR genes cause congenital hypothyroidism. Gene
mutations cause the loss of thyroid function in one
of two ways. Mutations in the PAX8 gene and some mutations
in the TSHR gene prevent or disrupt the normal development
of the thyroid gland before birth. Mutations in the
DUOX2, SLC5A5, TG, TPO, and TSHB genes prevent or reduce
the production of thyroid hormones, even though the
thyroid gland is present. Mutations in other genes that
have not been well characterized may also cause congenital
hypothyroidism.
Management
Most cases of congenital hypothyroidism are sporadic,
which means they occur in people with no history of
the disorder in their family.
An estimated 15 to 20 percent of cases are inherited.
Many inherited cases are autosomal recessive, which
means both copies of the gene in each cell have mutations.
Most often, the parents of an individual with an autosomal
recessive condition each carry one copy of the mutated
gene, but do not show signs and symptoms of the condition.
Some inherited cases (those with a mutation in the PAX8
gene or certain TSHR mutations) have an autosomal dominant
pattern of inheritance, which means one copy of the
altered gene in each cell is sufficient to cause the
disorder.
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