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Disorders of thyroid function in neonates often present a diagnostic dilemma. The initial signs and symptoms are often subtle or misleading. A good understanding of the unique thyroid physiology and the assessment of thyroid function is necessary to recognize, diagnose, and treat thyroid disorders.

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  1. Fetal and neonatal thyroid function

      1. Embryogenesis begins in the third week of gestation with thyroglobulin synthesis and continues through 10–12 weeks' gestation. At that time, thyroid-stimulating hormone (TSH) can be detected. Thyroid activity remains low until midgestation and then increases slowly until term.

      1. Thyroid hormones undergo rapid and dramatic changes in the immediate postnatal period.

          1. An acute release of TSH occurs within minutes after birth. Peak values of 80 mU/L are seen at 30–90 min. Levels decrease to <10 mU/L by the end of the first postnatal week.

          1. Stimulated by the TSH surge, thyroxine (T4), free T4 (FT4), and triiodothyronine (T3) rapidly increase, reaching peak levels by 24 h. Levels decrease slowly over the first few weeks of life.

      1. Thyroid function in the premature infant. Identical changes in TSH, T4, and T3 are seen in premature infants; however, absolute values are lower. TSH levels return to normal by 3–5 days of life regardless of gestational age.

  2. Physiologic action of thyroid hormones. Thyroid hormones have profound effects on growth and neurologic development. They also influence oxygen consumption, thermogenesis, and the metabolic rate of many processes. Maternal T4 is critical for normal central nervous system maturation in the fetus.

  3. Biochemical steps to thyroid hormone synthesis. Thyroid hormone production includes the stages of iodide transport, thyroglobulin synthesis, organization of iodide, monoiodotyrosine and diiodotyrosine coupling, thyroglobulin endocytosis, proteolysis, and deiodination.

  4. Assessment of thyroid function. Thyroid tests are intended to measure the level of thyroid activity and to identify the cause of thyroid dysfunction.

      1. T4 concentration is an important parameter in the evaluation of thyroid function. More than 99% of T4 is bound to thyroid hormone–binding proteins. Therefore, changes in these proteins may affect T4 levels. Serum levels for term newborn infants range between 6.4 and 23.2 mcg/dL.

      1. Free T4 reflects the availability of thyroid hormone to the tissues. Serum levels vary widely by gestational age: newborn term infants (2.0–5.3 ng/dL) and infants of 25–30 weeks' gestation (0.6–3.3 ng/dL).

      1. TSH measurement is a valuable test in evaluating thyroid disorders, particularly for primary hyperthyroidism. Serum levels over all gestational ages of 25–42 weeks range from 2.5–18.0 mU/L.

      1. T3 concentration is useful in the diagnosis and treatment of hyperthyroidism. Serum levels of T3 are very low in the fetus and cord blood samples (20–75 ng/dL). Shortly after birth, levels exceed 100 ng/dL to ~400 ng/dL. In hyperthyroid states, levels may exceed 400 ng/dL. In sick preterm infants, a very low T3 (hypothyroid range) may signal the euthyroid sick syndrome, also known as the nonthyroidal illness syndrome.

      1. Thyroid-binding globulin (TBG) can be measured directly by radioimmunoassay. T3 resin uptake provides an indirect measurement of TBG and is now considered ...

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