Endemic deficiency of dietary iodine is typically manifested by hyothryoidism early in childhood and has been also called congenital, but is not caused by genetic defects. It is a common cause of hypothyroidism in infants and children worldwide.
Autoimmune thyroid disease is a common cause of hypothyroidism in regions of the world where iodine is supplemented in dietary salt products. The vast majority of cases of autoimmune hypothyroidism are due to Hashimoto thyroiditis (discussed later).
Iatrogenic hypothyroidism can be caused by either surgical or radiation-induced ablation of thyroid parenchyma, or as an unintended adverse effect of certain drugs.
The clinical manifestations of hypothyroidism include cretinism and myxedema.
Cretinism refers to hypothyroidism developing in infancy or early childhood. This disorder formerly was
Table20.3CausesofHypothyroidism
PostablativeSurgery,radioiodinetherapy,orexternalirradiationAutoimmunehypothyroidism
Drugs(lithium,iodides,p-aminosalicylicacid) *Associatedwithenlargementofthyroid(“goitroushypothyroidism”).Hashimotothyroiditisandpostablativehypothyroidismaccountforthemajorityofcasesofhypothyroidismindevelopedcountries.
http://ebooksmedicine.net fairly common in areas of the world where dietary iodine deficiency is endemic, including mountainous areas such as the Himalayas and the Andes (endemic cretinism). It is now much less frequent because of the widespread supplementation of salt with iodine. By contrast, enzyme defects that interfere with thyroid hormone synthesis are a cause of sporadic cretinism. Clinical features of cretinism include impaired development of the skeletal system and central nervous system, severe mental retardation, short stature, coarse facial features, a protruding tongue, and umbilical hernia. The severity of the mental impairment in cretinism seems to be influenced by the time of onset of the deficient state in utero. Normally, maternal hormones that are critical to fetal brain development, including T3 and T4, cross the placenta. If maternal thyroid deficiency is present before the development of the fetal thyroid gland, mental retardation is severe. By contrast, reduction in maternal thyroid hormones later in pregnancy, after the fetal thyroid has developed, allows normal brain development.
• Hypothyroidism in older children and adults results in a condition known as myxedema. The initial symptoms include generalized fatigue, apathy, and mental sluggishness, which may mimic depression. Decreased sympathetic activity results in constipation and decreased sweating. The skin is cool and pale because of decreased blood flow. Reduced cardiac output contributes to shortness of breath and decreased exercise capacity, two frequent complaints. Thyroid hormones regulate the transcription of several sarcolemmal genes, such as calcium ATPases, whose encoded products are critical in maintaining efficient cardiac output. In addition, hypothyroidism promotes an atherogenic lipid profile—an increase in total cholesterol and low-density lipoprotein (LDL) levels—that may contribute to the adverse cardiovascular mortality rates in this disease. Histologically, there is an accumulation of matrix substances, such as glycosaminoglycans and hyaluronic acid, in skin, subcutaneous tissue, and viscera. This results in nonpitting edema, broadening and coarsening of facial features, enlargement of the tongue, and deepening of the voice.
The diagnosis of hypothyroidism is based on laboratory evaluation. As in the case of hyperthyroidism, measurement of serum TSH is the most sensitive screening test.