Type of test Blood
Normal findings
Adults: 42-110 ng/mL
Children:
Test explanation and related physiology
Growth hormone (GH) exerts its effects on many tissues through a group of peptides called somatomedins. The most commonly tested somatomedins are insulin-like growth factor 1 (IGF-1) and IGF-3. Measurement of free IGF-1 and IGF binding protein (BP) 3 is preferred to GH measurements in cases of short stature in early adolescence. IGF is the test of choice in identifying and monitoring treatment of acromegaly.
Great variation in GH secretion occurs during the day. A random GH assay result may significantly overlap between normal and abnormal values. To diminish the common variations in GH secretion, screening for IGF-1 provides a more accurate reflection of the mean plasma concentration of GH. Somatomedins are not affected (as GH is) by the time of day, food intake, or exercise because they circulate bound to proteins that are durable or long-lasting. Normally there is a large increase during the pubertal growth spurt.
Levels of IGF-1 depend on levels of GH. As a result, IGF-1 levels are low when GH levels are deficient. Nonpituitary causes of reduced IGF-1 levels include malnutrition, severe chronic illnesses, severe liver disease, hypothyroidism, renal failure, inflammatory bowel disease, and Laron dwarfism. Abnormally low-test results require an abnormally reduced or absent GH during a GH-stimulation test to make the diagnosis of GH deficiency.
Pediatricians commonly use IGF BPs to even further diminish the influence of the variables affecting GH and somatomedin levels. Specifically, IGF BP 2 and IGF BP 3 are the most commonly measured. However, if GH deficiency is strongly suspected yet documentation using GH or somatomedins is questionable, IGF BP determinations are helpful. IGF BP 3 is less age-dependent and is the most accurate (97% sensitivity and specificity). These proteins help to evaluate GH deficiencies and GH-resistant syndromes (e.g., Laron dwarfism). Finally, these binding proteins are very useful in predicting responses to therapeutic exogenous GH administration.
Interfering factors
* Drugs that may cause decreased levels include high doses of estrogens.
Procedure and patient care
• See inside front cover for Routine Blood Testing.
• Fasting: yes
• Blood tube commonly used: lavender or red
Abnormal findings
Increased levels
- Acromegaly
- Gigantism
- Hyperpituitarism
- Obesity
- Pregnancy
- Precocious puberty
Decreased levels
- GH deficiency/resistance
- Laron dwarfism
- Inactive GH
- Resistance to somatomedins
- Nutritional deficiency
- Delayed puberty
- Pituitary tumor
- Hypopituitarism
- Cirrhosis of the liver
