When treatment (with levothyroxine) is initiated within 1 month or less after birth, the prognosis for normal intellectual development is excellent; screening programs usually offer therapy within 1 to 2 weeks of birth. If therapy is instituted after 6 months, when the signs of severe hypothyroidism are present, the likelihood of normal intellectual function is markedly decreased. Growth improves after thyroid replacement even in late diagnosed cases. The dose of levothyroxine changes with age; 10 to 15 μg/kg of levothyroxine is used for a newborn, but about 3 μg/kg is used later in childhood. In neonatal hypothyroidism, the goal is to bring the serum free T4 rapidly into the upper half of the range of normal. Suppression of TSH is not seen and not necessary in all cases because such suppression may lead to excessive doses of levothyroxine.
The etiology of acquired hypothyroidism is presented in Table 175-2. The clinical manifestations may be subtle. Hypothyroidism should be suspected in any child who has a decline in growth velocity, especially if not associated with weight loss (see Table 175-3). The most common cause of acquired hypothyroidism in older children in the United States is lymphocytic autoimmune thyroiditis (Hashimoto thyroiditis). In many areas of the world, iodine deficiency is the etiology of endemic goiter (endemic cretinism). The failure of the thyroid gland may be heralded by an increase of TSH before T4 levels decrease. In contrast to untreated congenital hypothyroidism, acquired hypothyroidism is not a cause of permanent developmental delay.
Hashimoto thyroiditis. Also known as autoimmune or lymphocytic thyroiditis, Hashimoto thyroiditis is a common cause of goiter and acquired thyroid disease in older children and adolescents. A family history of thyroid disease is present in 25% to 35% of patients. The etiology is an autoimmune process targeted against the thyroid gland with lymphocytic infiltration and lymphoid follicle and germinal center formation preceding fibrosis and atrophy.
Clinical manifestations include a firm, nontender euthyroid, hypothyroid, or, rarely, hyperthyroid (hashitoxicosis) diffuse goiter with a pebble-like surface. See Table 175-3 for signs and symptoms, which can be diverse. Onset typically occurs after 6 years of age with a peak incidence in adolescence with a female predominance. Associated autoimmune diseases include diabetes mellitus type 1 (DM1), adrenal insufficiency, and hypoparathyroidism. Autoimmune polyglandular syndrome type I consists of hypoparathyroidism, Addison disease, mucocutaneous candidiasis and, often, hypothyroidism. Autoimmune polyglandular syndrome type II consists of Addison disease, DM1, and frequently autoimmune
Dull facies: thick lips, large tongue, depressed nasal bridge,
Sinus bradycardia/heart block Cold extremities Cold intolerance Pallor ECG changes: low-voltage QRS complex
Muscle weakness Hypotonia: constipation, potbelly Umbilical hernia Myxedema coma (carbon dioxide narcosis, hypothermia) Pseudohypertrophy of muscles Myalgia Physical and mental lethargy Developmental delay Delayed relaxation of reflexes Paresthesias (nerve entrapment: carpal tunnel syndrome) Cerebellar ataxia
Delayed bone age Epiphyseal dysgenesis, increased upper-to-lower segment ratio
Myxedema Serous effusions (pleural, pericardial, ascites) Hoarse voice (cry) Weight gain Menstrual irregularity Arthralgia Elevated CK Macrocytosis (anemia) Hypercholesterolemia Hyperprolactinemia Precocious puberty in severe cases
CK, Creatine kinase; ECG, electrocardiographic.