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الرئيسية » 🔬موسوعة التحاليل الطبية » Growth Hormone Stimulation (GH Provocation, Insulin Tolerance [IT], Arginine)

Growth Hormone Stimulation (GH Provocation, Insulin Tolerance [IT], Arginine)

Growth Hormone Stimulation (GH Provocation, Insulin  Tolerance [IT], Arginine)


NORMAL FINDINGS
GH levels >10 ng/mL or >10 mcg/L (SI units)
INDICATIONS
The GH stimulation test is used to identify patients who are suspected of having GH deficiency. A
normal patient can have low GH levels, but if GH is still low after GH stimulation, the diagnosis can be
more accurately made.



TEST EXPLANATION
Because GH secretion is episodic, random measurement of plasma GH is not adequate to make the diagnosis of GH deficiency. To diagnose GH deficiency, GH stimulation tests are indicated. One of the most reliable GH stimulators is insulin-induced hypoglycemia, in which the blood glucose declines to
less than 40 mg/dL. Other GH stimulants include vigorous exercise and drugs (e.g., arginine, glucagon, levodopa, clonidine). Glucagon is more widely used for GH stimulation because of safety concerns with insulin-induced hypoglycemia.
Pituitary GH deficiency cannot be diagnosed by identifying a deficiency of GH to just one stimulant, because as many as 20% of normal patients will fail to respond to the stimulant. Therefore a double stimulated test is usually performed: arginine infusion is followed by insulin-induced hypoglycemia.
Arginine is an amino acid that stimulates GH secretion; hypoglycemia also stimulates GH secretion. A GH concentration over 10 mg/L after stimulation effectively excludes GH deficiencies. Hypothyroidism should be excluded before GH stimulation testing.
GH also can be stimulated by vigorous exercise. This may entail running or stair-climbing for 20 minutes. Blood samples of GH are obtained at 0, 20, and 40 minutes. GH-releasing factor can also be used to stimulate GH. At present the best method of identifying patients deficient in GH is a positive stimulation test followed by a positive response to a therapeutic GH trial. GH deficiency is also sus￾pected when bone age, as determined by x-ray films of the long bones (see p. 1006), indicates delayed growth according to chronologic age.
Only minor discomfort is associated with this test and results from the insertion of the intravenous (IV) line and the hypoglycemic response induced by the insulin injection. This may include postural hypotension, somnolence, diaphoresis, and nervousness. This procedure is usually performed by a nurse under physician supervision. This test takes approximately 2 hours to perform.


CONTRAINDICATIONS
• Patients with epilepsy, because seizures can be induced by the hypoglycemia
• Patients with cerebrovascular disease, because hypoglycemia may induce stroke
• Patients with myocardial infarction (MI), because the stress associated with the hypoglycemia may cause angina or an MI
• Patients with low basal plasma cortisol levels, because they cannot respond to or compensate for the hypoglycemia

POTENTIAL COMPLICATIONS
• Hypoglycemia may be so significant and severe as to cause ketosis, acidosis, and shock. With close observation, this is unlikely.
PROCEDURE AND PATIENT CARE
Before
💠 Explain the procedure very carefully to the patient and, if appropriate, to the parents.
💠Instruct the patient to remain on nothing by mouth (NPO) status after midnight on the morning of
the test. Water is permitted.
During
• Note the following procedural steps:

  1. A saline lock IV line is inserted for the administration of medications and the withdrawal of frequent blood samples.
  2. Baseline blood levels are obtained for GH, glucose, and cortisol.
  3. Venous samples for GH are obtained 0, 60, and 90 minutes after injection of arginine and/or insulin or glucagon.
  4. Blood glucose levels are monitored at 15- to 30-minute intervals with the glucometer. The blood
    sugar should drop to less than 40 mg/dL for effective measurement of GH reserve.
    • Monitor the patient for signs of hypoglycemia, postural hypotension, somnolence, diaphoresis, and nervousness.
    • Ice chips are often given during the test for patient comfort.
    After
    • Observe the venipuncture site for bleeding.
    💢 Inform the patient and family that results may not be available for approximately 7 days. Some laboratories run GH tests only once a week.
    • After the test, give the patient cookies and punch or an IV glucose infusion.
    • Send the blood to the laboratory immediately after collection, because the half-life of growth hormone is only 20 to 25 minutes.

TEST RESULTS AND CLINICAL SIGNIFICANCE

⇩⇩⇩ Decreased Levels
Pituitary deficiency,
GH deficiency:
Diseases (e.g., tumor, infarction, trauma) of the pituitary can result in failure of the pituitary to secrete either GH or all the pituitary hormones. GH stimulation tests will fail to stimulate GHsecretion.

RELATED TESTS
Somatomedin C levels parallel GH levels. However, they are not affected by the many factors that cause significant variations in GH results.
Growth Hormone . This test is a direct quantitative assay for GH

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