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INTRODUCTION

Thyroid hormone is a potent regulator of metabolic rate and is essential to the function of most organ systems. In addition, the maintenance of normal thyroid status through childhood is necessary for normal growth and neurodevelopment. Optimal outcome in pediatric thyroid disease requires early detection and appropriate therapy with careful monitoring. These tasks are generally accomplished through successful collaboration between primary care providers and pediatric endocrine specialists. With an appropriate index of suspicion, pediatric thyroid disease often can be detected in early stages, and outcome is usually excellent. This chapter begins with a review of thyroid physiology and clinical testing. We then present diagnostic approaches to common pediatric thyroid disorders, grouped into the categories of hypothyroidism, thyrotoxicosis, and nodular thyroid disease.

THYROID PHYSIOLOGY

Embryonic Development

The fetal thyroid begins as a thickening of the pharyngeal floor at the base of the tongue (foramen cecum), which forms a diverticulum that descends caudally into the resting position of the mature thyroid gland. During this migration, the thyroglossal duct is formed that connects the pharyngeal floor to the thyroid bed. This thyroglossal duct normally involutes, and by embryonic day 50 the thyroid primordium has formed a bilobed structure. The cells of this primordium differentiate into thyroid follicular cells that synthesize thyroid hormone. Thyroid C cells, which produce calcitonin, have a distinct embryologic origin in the ultimobranchial glands.

By 10 to 12 weeks of gestation, the fetal thyroid is capable of concentrating iodine and synthesizing thyroxine (T4). Prior to this point, the human fetus is completely dependent on maternal thyroid hormone. Later in pregnancy, transplacental passage remains an important source of fetal thyroid hormone, as evidenced by the fact that T4 is detectable in the serum of infants born with complete thyroid agenesis.

The fetal relationship of thyrotropin (also called thyroid-stimulating hormone [TSH]) to T4 matures throughout gestation. Compared to maternal serum, fetal serum has lower concentrations of both T4 and triiodothyronine (T3). Birth is associated with a robust but transient peak in serum TSH, T4, and T3 (referred to as the neonatal surge), followed by rapid changes in the metabolism of thyroid hormone in peripheral tissues. This normal neonatal TSH surge typically lasts 1 to 2 days, which is the rationale for delaying newborn screening until 2 days after birth when practical.

Abnormal development of the thyroid (thyroid dysgenesis) is the most common cause of congenital hypothyroidism. As described further, the study of patients with thyroid dysgenesis has provided valuable insight into the molecular biology of normal thyroid development.

Thyroid Hormone Synthesis

The sole physiologic function of the thyroid gland is to synthesize T4 and T3. Dietary iodine is a critical nutritional requirement for the formation of these thyroid ...

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