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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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Table 527-1. Thyroid Disorders in Infancy and Childhood 

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).


Transient Primary Hypothyroidism


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. ...

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