Congenital hypothyroidism

Abstract

Congenital hypothyroidism (CH) occurs in approximately 1:2,000 to 1:4,000 newborns. The clinical manifestations are often subtle or not present at birth. This likely is due to trans-placental passage of some maternal thyroid hormone, while many infants have some thyroid production of their own. Common symptoms include decreased activity and increased sleep, feeding difficulty, constipation, and prolonged jaundice. On examination, common signs include myxedematous facies, large fontanels, macroglossia, a distended abdomen with umbilical hernia, and hypotonia. CH is classified into permanent and transient forms, which in turn can be divided into primary, secondary, or peripheral etiologies. Thyroid dysgenesis accounts for 85% of permanent, primary CH, while inborn errors of thyroid hormone biosynthesis (dyshormonogeneses) account for 10-15% of cases. Secondary or central CH may occur with isolated TSH deficiency, but more commonly it is associated with congenital hypopitiutarism. Transient CH most commonly occurs in preterm infants born in areas of endemic iodine deficiency. In countries with newborn screening programs in place, infants with CH are diagnosed after detection by screening tests. The diagnosis should be confirmed by finding an elevated serum TSH and low T4 or free T4 level. Other diagnostic tests, such as thyroid radionuclide uptake and scan, thyroid sonography, or serum thyroglobulin determination may help pinpoint the underlying etiology, although treatment may be started without these tests. Levothyroxine is the treatment of choice; the recommended starting dose is 10 to 15 mcg/kg/day. The immediate goals of treatment are to rapidly raise the serum T4 above 130 nmol/L (10 ug/dL) and normalize serum TSH levels. Frequent laboratory monitoring in infancy is essential to ensure optimal neurocognitive outcome. Serum TSH and free T4 should be measured every 1-2 months in the first 6 months of life and every 3-4 months thereafter. In general, the prognosis of infants detected by screening and started on treatment early is excellent, with IQs similar to sibling or classmate controls. Studies show that a lower neurocognitive outcome may occur in those infants started at a later age (> 30 days of age), on lower l-thyroxine doses than currently recommended, and in those infants with more severe hypothyroidism.

Figure 1

Infant with congenital hypothyroidism. A - 3 month old infant with untreated CH; picture demonstrates hypotonic posture, myxedematous facies, macroglossia, and umbilical hernia. B - Same infant, close up of face, showing myxedematous facies, macroglossia, and skin mottling. C - Same infant, close up showing abdominal distension and umbilical hernia.

Figure 2

Radiograph of the left lower extremity of two infants, showing absence of the distal femoral epiphysis on left. Radiograph of the left lower extremity of two infants. The infant on the left with congenital hypothyroidism demonstrates absence of the distal femoral and proximal tibial epiphyses, while in the normal infant on the right the distal femoral epiphysis is present.

Figure 3

Bamforth- Lazarus syndrome. An 8 month old infant with a homozygous mutation in the TTF-2 gene locus leading to congenital hypothyroidism. Phenotypic features include, low set ears, extensive cleft palate, hypertelorism, spiky hair and low posterior hairline. (Taken from; A novel loss-of-function mutation in TTF-2 is associated with congenital hypothyroidism, thyroid agenesis and cleft palate; Human Molecular Genetics, 2002, Vol. 11, No. 17. Courtesy Dr. Michel Polak and the Oxford University Press.)

Figure 4

Congenital hypothyroidism: Diagnostic algorithm. Diagnostic algorithm: the diagnosis of congenital hypothyroidism begins with either abnormal newborn screening test results or a clinical suspicion of hypothyroidism, leading to serum thyroid function tests (typically TSH and free T4) to confirm the diagnosis. If a diagnosis of primary or secondary (central) congenital hypothyroidism is confirmed, other diagnostic studies can be undertaken to determine the underlying etiology.

Figure 5

Technetium 99 m scan findings in congenital hypothyroidism. A-Technetium 99 m scan, showing a large gland (approximately twice normal size) in eutopic location, consistent with dyshormonogenesis. B-Technetium 99 m scan, showing uptake in ectopic location, i.e. ectopic gland. C-Minimal uptake, consistent with aplasia or severe hypoplasia.