Turnaround times
The quoted turnaround time is from sample receipt in the laboratory, to results authorisation in the Laboratory Information Management system. The times do not include transport of specimen to the laboratory or the administrative process to print and post/email reports. Service users must allow for transport and reporting time when ordering tests.
Clinical background:
Growth hormone (GH) is a polypeptide hormone secreted by the anterior pituitary gland. Secretion is episodic and is associated with exercise, the onset of deep sleep prandial status and in response to low glucose levels. Synthesis and release is under the control of hypothalamic releasing peptides and inhibitory peptides such as somatostatin. Insulin-like growth factor (IGF-1) exerts an inhibitory effect through negative feedback mechanisms. Circulating GH consists of several molecular isoforms, with 22000 Dalton GH being the most abundant, followed by a 20000 Dalton GH variant. GH is physiologically important in 2 main areas. Firstly, it has an integral role in skeletal growth which is demonstrated in either excess or deficiency in childhood. The action of GH in part is mediated through IGF-1 as well as promoting protein synthesis and the uptake of amino acids into cells. Secondly, GH influences intermediary metabolism by stimulating lypolysis and is antagonistic to the insulin mediated uptake of glucose. GH secretion is stimulated by hypoglycaemia, and suppressed by hyperglycaemia.
GH is commonly measured in the investigation of GH excess or insufficiency. Random samples may be of limited diagnostic utility due to the episodic nature of GH secretion. Diagnosis or exclusion of GH deficiency or excess often requires measurement of GH response to dynamic function testing, for example the oral glucose tolerance test in investigation of GH excess or stimulation tests in investigation of GH insufficiency.
Specimen container paediatric:
serum, lithium heparin plasma, EDTA plasma.
Specimen container adult:
serum, lithium heparin plasma, EDTA plasma.
Minimum volume paediatric:
1 mL blood
Minimum volume adult:
2 mL blood
Sample stability:
Unseparated: Same day
Separated:
7 days at 4 to 8°C,
2 months at -20°C
Transport requirements:
Ambient
Interpretation:
Provision of diagnostic cut-offs for GH during dynamic function testing is difficult due to a lack of normative data that are age, gender and BMI adjusted and also due to variability between GH assays. The following recommendations are made in the Endocrine Society Clinical Practice Guidelines:
Investigation of acromegaly:
Single random GH measurements are not recommended for diagnosis due to inherent episodic GH secretion from normal and adenomatous pituitary glands. A nadir serum GH concentration <1 µg/L within 2 hours of 75g oral glucose load usually excludes acromegaly. Increasing age and elevated BMI may be associated with abnormal post-glucose GH suppression.
Investigation of growth hormone insufficiency:
According to a multi-centre study cut-offs of 5.1 µg/L for the insulin tolerance test and 4.1 µg/L for the GHRH-arginine tests had sufficient specificity and sensitivity for the diagnosis of adult growth hormone deficiency. However, the appropriate cut-offs are related to BMI, with lower cut-offs required for those with higher BMIs compared to those with lower.
Reference ranges:
No reference ranges are provided due to the pulsatile nature of growth hormone secretion.
Factors affecting result:
Gross haemolysis, lipaemia or icterus may interfere.
Heterophilic antibodies can interfere with immunoassays.
Other info:
Lithium Heparin anticoagulated samples are acceptable