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Causes of hypothyroidism are listed
in Table 527-1. Transient neonatal hypothyroidism
occurs in premature infants, and may be caused by drugs or maternal
antibodies. The most common cause of nonendemic congenital hypothyroidism
is defective thyroid embryogenesis. Inborn defects in thyroid hormone synthesis
or action are the second most common cause of congenital hypothyroidism.1-13 Other
causes include intrauterine exposure to goitrogenic agents and hypothalamic-pituitary
disorders. These usually occur in the setting of panhypopituitarism,
but isolated defects in thyrotropin-releasing hormone (TRH) or thyroid-stimulating
hormone (TSH) do occur.5-7
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Premature infants are defined by gestational age (GA) and weight.
By weight, premature infants are classified as low birth weight
(LBW, 2500–1500 g), very low birth weight (VLBW, 1500–1000
g), and extremely low birth weight (ELBW, < 1000 g); they range
from 34 to 35 weeks’ to 23 to 24 weeks’ GA (the
current threshold of viability).3Most premature infants
have some degree of hypothyroxinemia (T4 < 6.5 μg/dL,
84 nmol/L). The prevalence of T4 values < 6.5 μg/dL
approximates 50% in VLBW infants. These infants have a
relatively immature hypothalamic-pituitary-thyroid axis, immature
metabolic systems, and high prevalence of neonatal morbidities,
including respiratory distress, hypoxia, undernutrition, gastrointestinal
and cardiac dysfunction, sepsis, and cerebral pathology. As result,
they are predisposed to development of transient primary
hypothyroidism and the syndrome of transient hypothyroxinemia
of prematurity (THOP), which probably represents transient
hypothalamic-pituitary hypothyroidism and/or nonthyroidal
illness (NTI; the low T3 syndrome).
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Transient Primary Hypothyroidism
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Transient primary hypothyroidism is characterized by low serum
T4 and high thyroid-stimulating hormone (TSH) concentrations.3 The
prevalence of transient hypothyroidism among premature neonates
depends on the extent of iodine deficiency in the environment. Term
infants can have transient neonatal hypothyroidism in areas of low iodine
intake (endemic goiter). In Belgium, approximately 20% of
premature infants have transient hypothyroidism. In North America,
the incidence is less than 1%, presumably due to higher
average iodine intake. Transient primary hypothyroidism develops
during the first 1 to 2 weeks of extrauterine life and is superimposed
on the usual state of transient hypothyroxinemia characteristic
of prematurity. Urinary ...