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Hossein Moravej

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Epidemiology The prevalence of congenital hypothyroidism is 1/4,000 infants worldwide. Twice as many girls as boys are affected.

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Etiology Thyroid dysgenesis (aplasia, hypoplasia, or an ectopic gland) is the most common cause of congenital hypothyroidism, accounting for 85% of cases; Inborn error of thyroxine synthesis (10%) Transplacental maternal thyrotropin-receptor blocking antibody (5%).

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Thyroid dysgenesis of cases of thyroid dysgenesis are aplasia, however, In the other of infants, rudiments of thyroid tissue are found in an ectopic location(hypoplasia).

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Thyroid dysgenesis Most cases of hypoplasia are detected by newborn screening, but in some children ectopic thyroid tissue (lingual, sublingual, subhyoid) may provide adequate amounts of thyroid hormone for many years, or it may eventually fail in early childhood.

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Defective Synthesis of Thyroxine (Dyshormonogenesis) Goiter is almost always present. When the defect is incomplete, compensation occurs, and onset of hypothyroidism may be delayed for years.

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Dyshormonogenesis Defect of Iodide Transport Thyroid peroxidase defects of organification and coupling: After iodide is trapped by the thyroid, it is rapidly oxidized to reactive iodine, which is then incorporated into tyrosine units on thyroglobulin. This process requires generation of H 2 O 2, thyroid peroxidase, and hematin

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Dyshormonogenesis Defects of Thyroglobulin Synthesis Defects in Deiodination: Monoiodotyrosine and diiodotyrosine released from thyroglobulin are normally deiodinated within the thyroid or in peripheral tissues by a deiodinase

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Defects in Thyroid Hormone Transport: Passage of thyroid hormone into the cell is facilitated by plasma membrane transporters

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Maternal thyrotropin receptor-blocking antibody An unusual cause of transitory congenital hypothyroidism. Transplacental passage of maternal TRBAb inhibits binding of TSH to its receptor in the neonate. It should be suspected whenever : history of maternal autoimmune thyroid disease, including Hashimoto thyroiditis or Graves disease, maternal hypothyroidism on replacement therapy, or recurrent congenital hypothyroidism of a transient nature in subsequent siblings.

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Maternal thyrotropin receptor-blocking antibody The half-life of the antibody is 21 days, and remission of the hypothyroidism occurs in about 3-6 mo

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Radioiodine administration Hypothyroidism can occur as a result of inadvertent administration of radioiodine during pregnancy for treatment of Graves disease or cancer of the thyroid.

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Thyrotropin and Thyrotropin-Releasing Hormone Deficiency Deficiency of TSH can occur in developmental defects of the pituitary or hypothalamus. More often, the deficiency of TSH is secondary to a deficiency of TRH.

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Most infants with congenital hypothyroidism are asymptomatic at birth, even if there is complete agenesis of the thyroid gland. It is due to the transplacental passage of moderate amounts of maternal T 4, which provides fetal levels that are approximately 33% of normal at birth.

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The clinician depends on neonatal screening tests for the diagnosis of congenital hypothyroidism. Laboratory errors occur, and awareness of early symptoms and signs must be maintained.

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Birthweight and length are normal, but head size may be slightly increased because of myxedema of the brain. Prolongation of physiologic jaundice, may be the earliest sign. Feeding difficulties, especially sluggishness, somnolence, and choking spells during nursing, are often present during the 1st mo of life. Respiratory difficulties, due in part to the large tongue, include apneic episodes, noisy respirations, and nasal obstruction.

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Cry little, sleep much, have poor appetites, and are generally sluggish. constipation that does not usually respond to treatment. Umbilical hernia Hypothermia