Congenital Hypothyroidism Workup
- Author: Maala S Daniel, MBBS; Chief Editor: Stephen Kemp, MD, PhD more...
Diagnosis of primary hypothyroidism is confirmed by demonstrating decreased levels of serum thyroid hormone (total or free T4) and elevated levels of thyroid-stimulating hormone (TSH).
If maternal antibody–mediated hypothyroidism is suspected, maternal and neonatal antithyroid antibodies may confirm the diagnosis. Such antibodies are an uncommon cause of congenital hypothyroidism.
Low or low-normal serum total T4 levels in the setting of a serum TSH within the reference range suggests TBG deficiency. This congenital disorder causes no pathologic consequence; however, it should be recognized to avoid unnecessary thyroid hormone administration. Thyroid-binding globulin (TBG) deficiency affects 1 individual per 3000 population; therefore, occurrence is nearly as frequent as that in congenital hypothyroidism. TBG deficiency results in low serum total T4 levels; however, serum TSH and serum-free T4 concentrations are normal. Assessment of the serum TBG concentration, preferably with simultaneous serum free and serum total T4 concentrations, confirms the diagnosis.
TBG levels can be measured in infants with suspected TBG deficiency. This condition does not require treatment, but appropriate diagnosis and parental counseling can avoid later confusion and misdiagnosis.
Routine laboratory testing in patients with TBG deficiency shows a low total T4 level and a TSH level within the reference range. Free T4 and T3 levels are within the reference range. Congenital nephrotic syndrome is a rare cause of TBG deficiency or congenital hypothyroidism.[52, 53]
Laboratory results similar to infants with TBG deficiency can be found in infants who have hypopituitarism or hypothalamic disease, but these children have normal TBG levels.
Thyroid scanning (using technetium-99m or iodine-123) may be useful in defining the cause of hypothyroidism and may aid in genetic counseling. It can aid in distinguishing congenital hypothyroidism from transient hyperthyrotropinemia.[54, 5] The absence of radionuclide uptake suggests sporadic athyreotic hypothyroidism but can also be seen when uptake is blocked by excess iodide or thyroid receptor blocking antibodies. If no uptake is found on isotope scanning, thyroid ultrasonography may demonstrate thyroid tissue in these patients.[5, 55] One study of 210 scanned infants stated a preference for using iodine-123 over pertechnetate.
Thyroid scans can also demonstrate the presence of an ectopic thyroid, such as a lingual or sublingual gland, which is also sporadic. The presence of a bilobed thyroid in the appropriate position or a goiter would suggest either an inborn error of thyroid hormone production or transient hypothyroidism or transient hyperthyrotropinemia. Thyroid scanning is not required to make or confirm the diagnosis of congenital hypothyroidism, but can provide important information about the etiology.
Ultrasonography may be a reasonable alternative or addition to scintigraphy but may fail to reveal some ectopic glands.[6, 57]
A lateral radiograph of the knee may be obtained to look for the distal femoral epiphysis. This ossification center appears at about 36 weeks' gestation. Its absence in a term or postterm infant indicates prenatal effects of hypothyroidism, and prior to the introduction of hormone measurements, was used as a diagnostic test for congenital hypothyroidism.
Early studies of outcome suggested that infants without a distal femoral epiphysis did less well than those with one, although both groups had results in the normal range. The author of this study was later unable to demonstrate an effect of bone age at diagnosis on outcome. Another study was unable to demonstrate any difference in outcome in infants with or without a distal femoral epiphysis.
Neonatal hypothyroidism screening, using TSH levels, has proven helpful in countries with mild to no iodine deficiency. It has not been found useful in countries with moderate-to-severe levels of iodine deficiency disorders (IDD) because resources are insufficient to deal with the problem, and efforts here should be made to supply sufficient iodine to the population as a whole.
In infants with suspected dyshormonogenesis, radioactive iodine uptake (iodine-123) and perchlorate flush testing (KCIO2) can be performed to determine the presence of an iodide uptake or organification defect.
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