26-13: Iodine Deficiency Disorder & Endemic Goiter

ESSENTIALS OF DIAGNOSIS

GENERAL CONSIDERATIONS

Mild to severe iodine deficiency exists in 30 countries; an estimated 1.9 billion people have insufficient iodine intake. Moderate iodine deficiency during gestation and infancy can cause manifestations of hypothyroidism, deafness, and short stature and lower a child’s intelligence quotient by 10–15 points. Even mild-to-moderate iodine deficiency appears to impair a child’s perceptual reasoning and global cognitive index. Severe iodine deficiency increases the risk of miscarriage and stillbirth. Cretinism occurs in about 0.5% of live births in iodine-deficient areas.

Although iodine deficiency is the most common cause of endemic goiter, there are other natural goitrogens, including certain foods (eg, sorghum, millet, maize, cassava), mineral deficiencies (selenium, iron, zinc), and water pollutants, which can themselves cause goiter or aggravate a goiter proclivity caused by iodine deficiency. In iodine-deficient patients, cigarette smoking can induce goiter growth. Pregnancy aggravates iodine deficiency. Some individuals are particularly susceptible to goiter owing to congenital partial defects in thyroid enzyme activity.

CLINICAL FINDINGS

A. Symptoms and Signs

Endemic goiters may become multinodular and very large. Growth often occurs during pregnancy, increasing the size of thyroid nodules and causing new nodules; compressive symptoms may occur.

Substernal goiters are usually asymptomatic but can cause tracheal compression, respiratory distress, dysphagia, superior vena cava syndrome, palsies of the phrenic or recurrent laryngeal nerves, or Horner syndrome. Cerebral ischemia and stroke can result from arterial compression or thyrocervical steal syndrome. Substernal goiters can rarely cause pleural or pericardial effusions. The incidence of significant malignancy is less than 1%.

Some patients with endemic goiter may become hypothyroid. Others may become thyrotoxic as the goiter grows and becomes more autonomous, especially if iodine is added to the diet.

B. Laboratory Findings

The serum T₄ and TSH are generally normal. TSH is low in hyperthyroidism if a multinodular goiter has become autonomous and there is sufficient iodine for thyroid hormone synthesis. TSH increases with hypothyroidism. Thyroid RAI uptake is usually elevated, but it may be normal if iodine intake has improved. Serum antithyroid antibodies are usually either undetectable or in low titers. Serum thyroglobulin is often elevated above 13 mcg/L. Urine iodine concentrations are low.

DIFFERENTIAL DIAGNOSIS

Endemic goiter must be distinguished from other forms of nodular goiter that may coexist in an endemic region.

PREVENTION

The daily minimum dietary requirement for iodine is 150 mcg daily in nonpregnant adults and 250 mcg daily for pregnant or lactating women. Iodine sufficiency is ...