Figure 175-1 Interrelationships of the hypothalamic-pituitary-thyroid axis. Thyroid-stimulating hormone (TSH) from the pituitary gland stimulates the secretion of thyroxine (T4) and triiodothyronine (T3) from the thyroid gland. These act at the pituitary gland level to control secretion of TSH by a negative feedback mechanism. In addition, T4 is metabolized to the potent T3 within the pituitary gland by a deiodinase. Secretion of TSH is stimulated by thyrotropin-releasing hormone (TRH) from the hypothalamus and inhibited by somatostatin. Thyroid hormone acts at the hypothalamus to stimulate secretion of somatostatin (somatostatin acts as a negative signal to the pituitary secretion of TSH). CRH, Corticotropinreleasing hormone; OB-R, leptin receptor. (From Melmed S, Polonsky K, Kronenberg H, Larsen R, editors: Williams Textbook of Endocrinology, ed 10, Philadelphia, 2003, Saunders, p 101.) of tyrosine; removing an iodine from the inner ring results in Serum TSH increases just after birth but soon decreases reverse T3, which has little biologic effect. Preferential conver-to lower values considered normal for later life. T4 secretion sion of T4 to reverse T3 rather than T3 occurs in utero and in increases after birth as a result of the peak in TSH and because all forms of severe illness, including respiratory distress syn-of maturation of thyroid metabolism. It is important to refer to drome, fevers, anorexia, cachexia, and starvation. Conversion age-adjusted normative data to interpret thyroid function tests from T4 to T3 increases immediately after birth and through-properly, whether relative to making diagnoses of hyperthyout life. T4 and T3 are noncovalently bound to a specific serum roidism or hypothyroidism or when adjusting therapy. Free T4 carrier protein, thyroxine-binding globulin, and, to a lesser is the test of choice because it eliminates the effects of variation extent, albumin. Only small (<0.02%) fractions of T4 and T3 in protein binding, which can be substantial. are not bound; free T4 (as it is converted to free T3) and free Table 175-1 summarizes laboratory test results in various T3 are biologically active. Free T3 exerts metabolic effects and types of thyroid abnormalities. In usual circumstances, plasma negative feedback on TSH release (Fig. 175-1). concentrations of TSH above the normal range indicate ↓, decreased; ↑, increased; N, Normal; T4, thyroxine; TBG, thyroxine-binding globulin; TRH, thyrotropin-releasing hormone; TSH, thyroid-stimulating hormone. *TSH may be slightly elevated.
primary hypothyroidism, and concentrations below the normal range most often indicate the presence of hyperthyroidism. Thyroid scans with 99m-pertechnetate or 123-I-iodine are occasionally indicated in the evaluation of pediatric thyroid disease. They can be useful in identifying thyroid agenesis or ectopic thyroid tissue.
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Short Stature Pubertal Delay Precocious Puberty in the Male Precocious Puberty in the Female
Hypothyroidism is diagnosed by a decreased serum free T4 and may be the result of diseases of the thyroid gland (primary hypothyroidism), abnormalities of the pituitary gland (secondary), or abnormalities of the hypothalamus (tertiary). Hypothyroidism is congenital or acquired and may be associated with a goiter (Table 175-2).