Definition
- Growth hormone (GH) is a 191-amino-acid polypeptide secreted by pituitary somatotrophs (about 50% of anterior pituitary endocrine cells).
- It promotes postnatal somatic growth (somatotropin) but does not stimulate fetal growth.
- Basal plasma GH (adults)0 to 5 ng/mL
Normal Values and Metabolism
GH half-life and IGF plasma levels
Parameter
GH half-life
- Value
- 6 to 20 minutes
GH bound to plasma protein
- Value
- About 50% (a plasma reservoir)
Plasma IGF-I
- Value
- 10 to 700 ng/mL (peaks at puberty)
Plasma IGF-II
- Value
- 300 to 800 ng/mL
- GH is metabolised rapidly by the liver.
- About 50% circulates bound to a plasma protein (a fragment of the GH receptor's extracellular domain), which acts as a reservoir and reflects tissue receptor number.
- Only a small amount of GH is excreted unaltered in urine, but daily urinary GH output reflects the 24-hour secretory profile.