Congenital Hypothyroidism
Introduction
Babies who are born with underactive thyroid function have a disorder
known as congenital hypothyroidism. The usual cause of this condition
is the failure of the thyroid gland to develop during pregnancy.
At birth the infants look normal and then slowly over a period of
weeks the clinical features of hypothyroidism appear. Because there
are no conspicuous signs or symptoms, the diagnosis of congenital
hypothyroidism is seldom made at birth by the examining physician.
If the condition goes unrecognized, which is usually the case, then
signs and symptoms such as constipation, dry skin, hoarse cry, large
tongue, swelling around the eyes, failure to suckle well, and prolonged
periods of sleep will appear within a few weeks.
Accompanying these overt clinical changes in the infant is the
less obvious damage to the brain resulting in mental retardation.
Although treatment at this juncture will reverse the clinical signs
and symptoms, the damage to the baby’s brain is irreversible.
The longer the disorder goes unrecognized (and thus untreated) the
greater the insult to the brain.
Fortunately, in recent years the burden of diagnosing congenital
hypothyroidism in the newborn period has been shifted from the physician
to the laboratory. In the early nineteen-seventies a French-Canadian
physician, Jean Dussault, found a way to measure thyroid hormone
(T4) in a tiny amount of dried blood in filter paper. The filter
paper was the same type that had been used for years to collect
blood from newborns before discharge from the hospital. The dried
blood specimens were sent to public health laboratories where tests
were done for PKU. and other metabolic diseases. Thus, this pre-existing
network for collecting newborn blood specimens provided the framework
upon which the same public health laboratories could not only screen
for metabolic diseases but could screen for congenital hypothyroidism
as well. It was not long before most industrialized nations had
established newborn screening programs to identify infants born
with hypothyroidism. Without question, the screening process has
revolutionized the diagnosis and early treatment of congenital hypothyroidism
and thereby prevented countless number of children from becoming
mentally retarded.
The Screening Process
Every child born in North America has blood collected on filter
paper by heel stick before discharge from the hospital or birthing
center. The dried blood specimens are forwarded to a central laboratory
where a one-eighth inch paper blood spot is tested for the amount
of T4 and/or thyroid stimulating hormone (TSH). A low T4 and elevated
TSH indicate that an infant lacks normal thyroid function. Although
an elevated TSH is a more sensitive and specific marker of hypothyroidism,
the majority of North American screening programs use T4 as the
initial test and confirm the diagnosis with the measurement of TSH.
Incidence And Types
Approximately one baby with congenital hypothyroidism is born out
of 3,500 births. However, it is estimated that 15-20% of hypothyroid
infants have a temporary form of the disorder and will only require
treatment for a limited number of years. When there is a question
of permanence of the disorder, the physician should discontinue
treatment after 3 years of age and repeat the blood tests for TSH
and T4 in a few weeks to be sure they are now normal.
There are three different types of thyroid abnormalities that are
associated with congenital hypothyroidism. Approximately 40% of
infants have under-developed or absent thyroid glands; 40% have
thyroid glands that are in the wrong place, such as under the tongue
or at the far side of the neck. The remaining 20% are unable to
manufacture thyroid hormone because of defects within the thyroid
gland. The latter condition is usually familial in nature.
Low T4 Values
Only a tiny number of infants with low T4 levels actually have
congenital hypothyroidism. More than 90% of low T4 values (not accompanied
by elevated TSH levels) in neonates are associated with other conditions
such as prematurity, low birth weight, illness, a deficiency in
the protein that carries T4 to various body tissues, or pituitary
failure. The latter is a rare disorder in which the pituitary gland
is unable to secrete TSH. This condition should be considered in
an infant whose growth rate falls off its expected growth curve
and has a low T4 and a low or normal TSH.
Treatment
The infant with suspected hypothyroidism should be seen without
delay by a physician, preferably by a pediatric endocrinologist.
Blood should be obtained to confirm the diagnosis, and treatment
with thyroxine should begin before the confirmatory T4 and TSH values
are available. Measurements of T4 and TSH should be made 2 and 4
weeks after starting the thyroxine, 2 weeks after a dosage change,
and every 1 to 2 months during the first year of life. Optimal intellectual
outcome depends on maintaining the circulating level of T4 in the
upper half of the normal range (10-16 µ g/dl) during the first
year of life.
If the T4 value is not above 10µ g/dl and the TSH is below
20 mU/l within 2 to 4 weeks after starting therapy, the physician
should consider the possibility that the baby has not been receiving
the medication or that there is a problem with absorption of T4
(soy-based formula or iron supplement will interfere with absorption
of T4).
Outcome
After recovering from the emotional shock of learning that their
baby has congenital hypothyroidism, the first question invariably
asked by the parents is, "Will our baby be normal?" Many
parents confuse congenital hypothyroidism with cretinism and harbor
the vision of their child becoming dwarfed and mentally retarded.
It is gratifying to be able to reassure parents that their children
will not be deficient in brain power and that the children are not
cretins. Severe iodine deficiency, the cause of cretinism in many
parts of the world, has not been a problem in the United States
since the introduction of iodized salt in the 1920s. Unfortunately,
mild degrees of iodine deficiency are now beginning to reappear
in the United States. These changes are described in our articles
about iodine elsewhere on this website.
When treatment is initiated before the appearance of clinical signs
and symptoms of hypothyroidism, the intellectual outcome of the
child is the same as that of children with normal thyroid function.
This was shown by a group of New England hypothyroid children whose
results in intelligence tests and in school performance were the
same as those of their unaffected classmates.
It is prudent to remember that as children grow older they have
a tendency to neglect their medication. This is especially true
for adolescents. A recent study in teenagers found a significant
decline in IQ that returned to normal after the appropriate blood
levels of T4 were restored. Consider non-compliance with medication
when school performance undergoes a change for the worse in your
adolescent child who is on treatment for hypothyroidism.
Conclusion
There are few instances in the practice of medicine where the health
and welfare of future generations can be positively affected; early
treatment of congenital hypothyroidism through newborn screening
is one of those instances.
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