Disorders of thyroid function in neonates often present a diagnostic dilemma. The initial clinical signs and symptoms are often subtle or misleading. A good understanding of the unique thyroid physiology, the assessment of thyroid function, and a sense of urgency are necessary to recognize, diagnose, and treat thyroid disorders early.
I. FETAL AND NEONATAL THYROID FUNCTION
Embryogenesis begins in the third week of gestation, with thyroglobulin synthesis detected by 4–6 weeks, thyrotropin-releasing hormone (TRH) synthesis by 6–8 weeks, and iodine trapping at 8–10 weeks through 12 weeks' gestation. At that time, thyroxine (T4), triiodothyronine (T3), and thyroid-stimulating hormone (TSH) secretion can be detected. Thyroid activity remains low until midgestation and then increases slowly until term.
Thyroid hormones undergo rapid and dramatic changes in the immediate postnatal period.
An acute release of TSH occurs within minutes after birth. Peak values of 60–80 mU/L are seen at 30–90 minutes attributed to clamping of the cord and the stress of delivery. Levels decrease to <10 mU/L by the end of the first postnatal week.
Stimulated by the TSH surge, T4, free T4 (FT4), and T3 rapidly increase, reaching peak levels by 24 hours. Levels decrease slowly over the first 1–2 weeks of life to levels typically seen in the infant.
Thyroid function in the premature infant. Identical changes in TSH, T4, and T3 are seen in premature infants; however, absolute values are lower in proportion to the gestational age and birthweight. TSH levels return to normal by 3–5 days of life.
II. 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.
III. 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.
IV. ASSESSMENT OF THYROID FUNCTION
Thyroid tests are intended to measure the level of thyroid activity and to identify the cause of thyroid dysfunction.
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.
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).
TSH measurement is a valuable test in evaluating thyroid disorders, ...