Thursday, April 3, 2014

Thyroid Hypofunction - Congenital Hypothyroidism (Cretinism)

Hypothyroidism is one of the most common endocrine problems of childhood. It may be either congenital, such as in cretinism, or acquired, such as from autoimmunization (Hasimotos's thyroditis). Hypothyroidism from dietary insufficiency of iodine is now rare because the use of iodized salt has permitted a readily available source of the nutrient.
Congenital hypothyroidism (cretinism)
Cretinism is usually caused by failure of embryonic development of the thyroid gland, but it may also be a result of inborn enzymatic defects in the synthesis of thyroxine. The severity of the disorder depends on the mount of thyroid tissue present. Usually the neonate does not exhibit obvious signs of hypothyroidism, probably because of the exogenous source of thyroid hormone supplied by means of the maternal circulation. Manifestations are delayed in breast-fed infants.
In another type of cretinism, transfer of goitrogens (substances that can induce a goiter), such as the antithyroid drugs phenylbutazone, para-aminosalicylic acid. And cobalt may inhibit thyroid secretion, thereby resulting in congenital cretinism. Although the latter is self-limiting, it is a potentially fetal condition because once the maternal supply is terminate, the infant's thyroid is unable to produce its own hormones. In addition, a large goiter in a neonate may cause total obstruction of the airway.
Clinical manifestations of congenital hypothyroidism
The symptoms of cretinism usually become apparent by 3 to 6 months of age in bottle-fed infants, However, before this time the earliest symptoms indicating hypothyroidism include prolonged physiologic jaundice, feeding difficulties, inactivity (excessive sleeping and minimal crying), anemia and problems resulting from hypotonic abdominal musculature, such as constipation, diastasis recti, protruding abdominal and umbilical hernia. The behavioral characteristics often lead parents to describe the infant as exceptionally "quiet and good"
Impaired development of the nervous system leads to mental retardation. The severity of the intellectual deficit is related to degree of hypothyroiditis, and the duration of the condition before treatment. Other nervous system manifestations include slow, awkward movements, somnolence, lethargy and abnormal deep tendon reflexes often referred to as "hung-up" because the relaxation phase after the contraction is slow.
Because skeletal growth is severely stunted, the child is short. Unlike pituitary dwarfism, infantile proportions persist in that the length of the trunk remains in weight gain and often leads to obesity. Characteristic infantile facial features from myxedema include a short forehead, wide, puffy eyes, wrinkled eyelids, broad, short, upturned nose, and a large protruding tongue. The hair is often dry, brittle, or lusterless and follows a low hairline. Dentition is delayed and usually defective. Such facial featured give the child a characteristic dull expression. The skin is yellowish from carotenemia as a result of the depressed hepatic conversion of carotene to vitamin A. Loss of heat from reduced metabolism is reflected in a cool skin. Cold intolerance is another common consequence. Anemia results in pallor, fatigue, and lethargy, and vitamin A deficiency causes thickened, coarse dry, scaly skin.
The cardiovascular changes are slow pulse, decreased circulation, mottling, and decreased pulse pressure. The decreased cardiac rate and output are directly related to the decreased oxygen requirements from a low metabolic rate. Respiratory changes include exertional dyspnea and decreased respiratory effort.
In breast-fed infants, the clinical manifestations may be delayed until the child is weaned, at which time the facial features, skin and hair changes, growth retardation, muscular hypotonia, and cardiovascular alterations becomes evident. Because breast milk contains suborptimum amounts of thyroid hormone, bone age is greatly retarded, usually comparable to that of a newborn. Significantly, however, intellectual functioning remains near normal.
Diagnostic evaluation
Several tests are available to assess thyroid activity:
• Measurement of protein-bound iodine (PBI)
• Measurement of free thyroxine
• Measurement of thyroid-stimulating hormone
• Measurement of thyrotropin-releasing factor, radioimmunoassay of thyroxine and triiodothyronine.
These tests measure the amount of thyroid hormone secreted and the intactness of the homeostatic mechanisms. Tests of thyroid gland function usually involve an oral infusion of a radioactive isotope of iodine-131 is decreased. Neonatal screening is now possible with a highly sensitive and specific radioimmunoassay for thyroxine and throid-stimulating factor and a low level of thyroxine during the early days of life. Roentgenography is employed to assess bone age.
Therapeutic management
Treatment involves indefinite replacement therapy with desiccated thyroid to abolish all signs of hypothyroidsm and re-establish normal physical and mental development. If adequate thyroid hormone replacement begins before 3 months of age, the chance for a normal intelligence quotient is increased. To avoid the risk of over-dosage of thyroid hormones, regular evaluations of thyroxine and triiodothyronine levels should be assessed. Bone age surveys are also done to ensure optimum growth.
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