Growth Hormone (GH, Human Growth Hormone [HGH], Somatotropin Hormone [SH])
Growth Hormone (GH, Human Growth Hormone [HGH],
Somatotropin Hormone [SH])
<5 ng/mL or <5 mcg/L (SI units)
Women: <10 ng/mL or <10 mcg/L
Newborn: 5-23 ng/mL (mcg/L [SI units])
1 week: 2-27 ng/mL (mcg /L [SI units])
1-12 mos: 2-10 ng/mL (mcg/L [SI units])1
year female: 0-10 ng/mL (mcg/L [SI units])
1 year male: 0-6 ng/mL (mcg/L [SI units])
This test is used to identify GH deficiency in adolescents with short stature, delayed sexual maturity, or other growth deficiencies. It is also used to document the diagnosis of GH excess in patients with gigantism or acromegaly. GH is used to identify and follow patients with ectopic growth hormone produced by neoplasms. Finally, it is often used as a screening test for pituitary hypofunction.
TEST EXPLANATION “
GH, or somatotropin, is secreted by the acidophil cells in the anterior pituitary gland and plays a central role in modulating growth from birth until the end of puberty. There is an elaborate feedback mechanism associated with the secretion of GH. The hypothalamus secretes growth hormone–releasing hormone, which stimulates GH release from the pituitary. GH exerts its effects on many tissues through a group of peptides called somatomedins. The most commonly tested somatomedin is somatomedin C , which is produced by the liver and has a major effect on cartilage. High levels of somatomedins stimulate the production of somatostatin from the hypothalamus. Somatostatin inhibits further secretion of GH from the pituitary. GH is secreted during sleep, exercise, and ingestion of protein and in response to hypoglycemia.
In the total absence of GH, linear growth occurs at one half to one third of the normal rate. GH also plays a role in the control of body anabolism throughout life by increasing protein synthesis, increasing the breakdown of fatty acids in adipose tissue, and increasing the blood glucose level.
If GH secretion is insufficient during childhood, limited growth and dwarfism may result. Also, a delay in sexual maturity may occur in adolescents with reduced GH levels. Conversely, overproduction of GH during childhood results in gigantism, with the person reaching nearly 7 to 8 feet in height. An excess of GH during childhood (after closure of long bone end plates) results in acromegaly, which is characterized by an increase in bone thickness and width but no increase in height.
GH tests are also used to confirm hypopituitarism or hyperpituitarism. GH assay is the most widely used test for GH deficiency or excess. Because GH secretion is episodic, random assays for GH are not adequate determinants of GH deficiency. Normal GH levels overlap significantly with deficient levels.
Low GH levels may indicate deficiency or may be normal for certain individuals at certain times of the day. To negate time variables in GH testing, GH should be drawn 60 to 90 minutes after deep sleep.
Levels increase during sleep. Also, strenuous exercise can be performed for 30 minutes in an effort to stimulate GH production. GH levels drawn at the end of the exercise period are expected to be maximal. These two methods are helpful in evaluating GH deficiency.
To negate the common variations in GH secretion, screening for insulin-like growth factor (IGF-1) or somatomedin C provides a more accurate reflection of the mean plasma concentration of GH. If the IGF-1 is normal, the patient virtually never has acromegaly. IGF-1 is not helpful in evaluating patients with GH deficiency because levels are affected by nutritional status, liver and thyroid function, and age.
These proteins are not affected by the time of day or food intake as is GH, because they circulate bound to proteins that are durable and long lasting.
A GH stimulation test can be performed to evaluate the body’s ability to produce GH in cases of suspected GH deficiency. The growth hormone suppression test is used to identify gigantism in children or acromegaly in the adult. If GH can be suppressed to less than 2 ng/mL, neither of these conditions exists. The most commonly used suppression test is the oral glucose tolerance test . Through a rise in glucose, GH normally is suppressed. In acromegalic patients, only a slight or no decrease in GH occurs.
• Random measurements of GH are not adequate determinants of GH deficiency, because hormone secretion is episodic.
• A radioactive scan performed within the week before the test may affect test results if levels are determined by radioimmunoassay (RIA).
• GH secretion is increased by stress, exercise, diet, and low blood glucose levels.
➢ Drugs that may cause increased levels include amphetamines, arginine, dopamine, estrogens, glucagon, histamine, insulin, levodopa, methyldopa, and nicotinic acid.
➢ Drugs that may cause decreased levels include corticosteroids and phenothiazines.
PROCEDURE AND PATIENT CARE
➢ Explain the procedure to the patient.
• The patient should not be emotionally or physically stressed, because this can increase GH levels.
• It is preferred that patients be well rested and are kept on nothing by mouth (NPO) status after midnight the morning of the test. Water is permitted.
• Collect a venous blood sample red-top tube.
• Because approximately two thirds of the total production of GH occurs during sleep, its secretion also can be measured during hospitalization by obtaining blood samples while the patient issleeping.
Growth Hormone Suppression Test
• Obtain peripheral venous access with normal saline solution (NSS).
•Obtain baseline GH and glucose levels as described above.
• Administer the prescribed glucose dose.
• Obtain GH and glucose levels at 10, 60, and 120 minutes after glucose ingestion.
• Apply pressure or a pressure dressing to the venipuncture site.
• Assess the venipuncture site for bleeding.
• Indicate the patient’s fasting status and the time the blood is collected on the laboratory slip. Include the patient’s recent activity (e.g., sleeping, walking, eating).
• Because the half-life of GH is only 20 to 25 minutes, send the blood to the laboratory immediately after collection.
TEST RESULTS AND CLINICAL SIGNIFICANCE
⇧⇧⇧ Increased Levels
These two syndromes are caused by excess GH.
Anorexia nervosa: Starvation stimulates GH secretion.
These situations stimulate GH secretion.
Hypoglycemia: Hypoglycemia stimulates GH secretion.
⇩⇩⇩ Decreased Levels
GH is produced in the pituitary. Diseases, tumors, ischemia, or trauma to the pituitary or hypothalamus GH deficiency.
Dwarfism: This is a result of GH deficiency in children.
Hyperglycemia: Elevated glucose levels inhibit GH secretion.
Failure to thrive: This is a result of GH deficiency in infants.
Delayed sexual maturity: This is a result of GH deficiency in adolescents.
Growth Hormone (GH) Stimulation . This is a test designed to stimulate the secretion of GH. It is required to accurately diagnose GH deficiency.