A single fasting growth hormone (GH) level is of limited value. The secretion of GH is episodic and pulsatile. GH has a half-life of 20 to 25 minutes. Testing for growth hormone deficiency or excess is best done as part of a dynamic test involving specific stimuli (see the online Endocrine Appendices: Growth Hormone Stimulation and Growth Hormone Suppression). Insulin-like growth factor-1 can also be useful in assessing growth hormone status.
Random growth hormone sampling should be performed on fasting patients who have rested for at least 30 minutes prior to collection. Growth hormone stimulation and suppression protocols are described in the online Endocrine Appendices: Growth Hormone Stimulation and Growth Hormone Suppression.

Reference:

Evaluation and Treatment of Adult Growth Hormone Deficiency: An Endocrine Society Clinical Practice Guideline