Background

Insulin-like growth factor I (IGF-I) is the effector of growth induced by growth hormone (GH). IGF-I deficiency can be the result of GH resistance or insensitivity due to genetic disorders of the GH receptor causing GH receptor deficiency (GHRD, Laron syndrome) or postreceptor defects, including the principal transduction agent STAT5b, the IGF-I/IGFBP3 stabilizer acid labile subunit (ALS), the IGF-I gene, or the IGF-I receptor.Acquired forms of GH insensitivity include the rare GH1 mutation (in which GH inhibiting antibodies develop after a few months of replacement therapy with recombinant GH) and, far more commonly, malnutrition, hepatic disease, renal disease, and diabetes.

Pathophysiology

The GH molecule binds to its specific cell surface receptor (GHR), which dimerizes with another GHR molecule so that the single GH molecule is enveloped by 2 GHR molecules. The intact receptor lacks tyrosine kinase activity, but binding of GH and dimerization results in association with JAK2, a member of the Janus kinase family, which results in self-phosphorylation of the JAK2 and a cascade of phosphorylation of cellular proteins. The most critical of these proteins is the signal transducer and activator of transcription 5b (STAT5b), which couples GH binding to the activation of gene expression that leads to the intracellular effects of GH, including synthesis of IGF-I, insulin-like growth factor binding protein 3 (IGFBP3), and ALS.

Hepatic IGF-I circulates almost entirely bound to IGF binding proteins (IGFBPs), with less than 1% being free. The IGFBPs are a family of 6 structurally related proteins with a high affinity for binding IGF. The principal BP, IGFBP3, binds approximately 90% of circulating IGF-I in a large (150-200 kD) ternary complex consisting of IGFBP3, ALS, and the IGF molecule. The ALS stabilizes the IGF–IGFBP3 complex, reduces the passage of IGF-I to the extravascular compartment, and extends its half-life.

IGF binding involves 3 types of receptors: the structurally homologous insulin receptor and type 1 IGF receptor and the distinctive type 2 IGF-II/mannose-6-phosphate receptor. Although the insulin receptor has a low affinity for IGF-I, IGF-I is present in the circulation at molar concentrations that are 1000 times those of insulin. Thus, even a small insulin-like effect of IGF-I could be more important than that of insulin itself, were it not for the IGFBPs that control the availability and activity of IGF-I. In fact, intravenous infusion of recombinant human IGF-I (rhIGF-I) can induce hypoglycemia, especially in the IGFBP3 deficient state.

Clinical Presentation

The clinical features of GHRD are not different than those of severe GH deficiency. Postreceptor Continue reading »

• Disturbances of growth are the most common presenting complaints in the pediatric endocrine clinic.
  • Fetal growth is dependent on maternal factors (placental sufficiency, maternal nutrition, etc.), insulin-like growth factor-2 (IGF-2) and insulin.
  • Growth in late infancy and childhood is dependent on growth hormone/IGF-1 axis and thyroid hormone. Growth is more rapid during infancy—up to 20 cm per year. It is common to see shifts in the growth curve in the first 18 months when children are adjusting to their genetic potential growth isopleth. During childhood, growth rate is fairly constant at approximately 2 inches (approximately 5 cm) per year.
  • Pubertal growth is dependent on sex hormones as well as growth hormone/IGF-1 axis and the thyroid gland. There is a mild deceleration in growth velocity before initiation of pubertal growth spurt.
• Abnormal growth and stature: criteria
  • Child’s growth curve is crossing percentiles.
  • Child’s growth rate is <2 inches or 5 cm per year.
  • Height is >2 standard deviations (SDs) (4 inches/10 cm) below from midparental height.
• If poor weight gain and lack of nutrition is the problem without affecting height velocity, it is unlikely to be an endocrine cause and patient may warrant a gastrointestinal evaluation instead.

The serum calcium level is low (5–7 mg/dL), and the phosphorus level is elevated (7–12 mg/dL).

Blood levels of ionized calcium (usually approximately 45% of the total) more nearly reflect physiologic adequacy but also are low.

The serum level of alkaline phosphatase is normal or low, and the level of 1,25[OH]₂D₃ is usually low, but high levels have been found in some children with severe hypocalcemia.

The level of magnesium is normal but should always be checked in hypocalcemic patients.

Levels of PTH are low when measured by immunometric assay.

Administration of the synthetic 1–34 fragment of human PTH (teriparatide acetate) results in increased urinary levels of cyclic adenosine monophosphate and phosphate. This response differentiates hypoparathyroidism from pseudohypoparathyroidism. With the advent of very sensitive PTH assays, this test is usually not necessary.

Radiographs of the bones occasionally reveal an increased density limited to the metaphyses, Continue reading »