Chapter 45: Hypothyroidism

SCOPE

DISEASE/CONDITION(S)

Fetal and neonatal hypothyroidism.

GUIDELINE OBJECTIVE(S)

Review the clinical significance of fetal and neonatal hypothyroidism, review causes of fetal and neonatal hypothyroidism, and present recommendations for diagnosis and management of fetal and neonatal hypothyroidism.

BRIEF BACKGROUND

Thyroid hormone is essential for normal growth and development. Because thyroid hormone plays a critical role in brain development from fetal life through 2–3 years of age, hypothyroidism during this period can cause abnormal neurodevelopment. Worldwide, hypothyroidism in infancy is the most common cause of preventable mental retardation in children, but neurocognitive deficits can be avoided by prompt and adequate treatment. Since the 1970s, the implementation of universal newborn screening for hypothyroidism has essentially eliminated hypothyroidism as a cause of severe mental retardation in many parts of the world. Nevertheless, with an incidence of around 1:2000 births, congenital hypothyroidism poses a significant risk to many infants, and care is needed to ensure early diagnosis and initiation of therapy.

Preterm and low birth weight (LBW) infants are at increased risk of thyroid function abnormalities due to many factors, including immaturity of the hypothalamic–pituitary–thyroid (HPT) axis, systemic illness (e.g., cardiovascular disease, respiratory distress syndrome, sepsis), medications that affect thyroid function (e.g., dopamine, glucocorticoids), and iodine excess or deficiency. Very low birth weight (VLBW) infants have a much higher incidence of hypothyroidism (up to 1:250) than do normal weight infants, and low levels of thyroxine (T4) are observed in up to 50% of infants born prior to 28 weeks. Hypothyroxinemia is correlated with poor medical and developmental outcomes in preterm infants. While a randomized trial suggested a possible benefit of treatment on short-term neurodevelopment in infants born under 27 weeks, subsequent studies have failed to show a clear benefit on long-term developmental outcomes. Hypothyroidism in preterm/LBW infants may manifest later than in term infants, so repeat screening at 2–4 weeks of age is indicated in this population.

Nearly all T4 circulates bound to plasma proteins, primarily thyroxine-binding globulin (TBG), but only unbound free T4 (fT4) is able to exert biological effects. Therefore, direct measurement of fT4 is desirable when assessing for hypothyroidism. However, many current fT4 assays can be confounded by acute illness, medications, or abnormalities of binding proteins that are present in many hospitalized patients. In such cases, measurement of total T4 may be helpful but should always be accompanied by measurement of protein binding such as a thyroid hormone binding ratio or T3 resin uptake, which can be used calculate the free T4 index (an estimate of fT4).

RECOMMENDATIONS

MAJOR RECOMMENDATIONS

1. Maternal history of thyroid disease should be elicited in all cases to assess risk factors for fetal or neonatal thyroid dysfunction.

2. Fetal goiter may be assessed by ultrasound. If present and causing airway compression, ...