When is the best time to draw a serum human growth hormone (hGH) sample?

Best Time to Draw Human Growth Hormone (hGH)

For diagnostic evaluation of GH deficiency, draw blood samples during a stimulation test at standardized intervals (typically at -30,0,30,60,90,120, and 150 minutes), with the test scheduled in the morning following an overnight fast and at least 30 minutes of bed rest. 1

Diagnostic Testing Context

Stimulation Testing (Standard Approach)

• Schedule the test in the morning after a normal night's sleep, with the overnight fast continuing through the test period 1
• Place patients at bed rest for at least 30 minutes before infusion begins to minimize stress-related GH fluctuations 1
• Blood samples should be drawn at -30,0,30,60,90,120, and 150 minutes during arginine stimulation testing 1
• Samples should be promptly centrifuged and plasma stored at -20°C until assayed 1

Spontaneous GH Secretion Monitoring

• Peak GH secretion occurs 4.5 minutes after the onset of stage 3 or 4 sleep, with peripheral GH concentrations peaking approximately 16 minutes after sleep onset 2
• The major GH peak (13-72 ng/mL) appears with the onset of deep sleep and lasts 1.5-3.5 hours 3
• Nocturnal sampling during the first 6 hours after sleep onset captures the physiologically relevant GH secretion pattern 4, 5
• Blood should be drawn every 30 minutes during the first 6 hours of sleep for comprehensive assessment 4

Key Physiological Considerations

Circadian Rhythm Effects

• GH exhibits marked diurnal variation, with highest concentrations during nocturnal hours 6
• In humans (diurnal species), GH peaks during the night, specifically during early sleep phases 6
• Random daytime GH measurements are diagnostically unreliable due to pulsatile secretion patterns and low baseline values between pulses 2, 3

Sleep-Dependent Secretion

• Approximately 70% of daily GH secretion occurs during the first deep sleep cycle 3
• Peak GH secretion is delayed if sleep onset is delayed, demonstrating the tight coupling between sleep and GH release 3
• If subjects are awakened for 2-3 hours and return to sleep, another GH peak (14-46 ng/mL) occurs 3

Clinical Application by Patient Population

Children with Suspected GH Deficiency

• IGF-1 measurement (age-adjusted, sex-adjusted, and Tanner stage-matched) should be obtained first as a screening tool 6
• If IGF-1 is elevated, proceed with oral glucose tolerance test; GH should suppress below 1 μg/L in normal individuals (though this can be difficult to achieve in normal adolescence) 6
• Both GH and IGF-1 should be monitored at baseline and during follow-up, as baseline GH levels predict surgical outcomes in cases of GH excess 6
• The screening test measuring GH peak during the first 2 hours after falling asleep correlates well with 6-hour nocturnal profiles (r=0.94) but not with stimulation tests 4

Timing Pitfalls to Avoid

• Do not draw random daytime samples for diagnostic purposes, as GH secretion is pulsatile with long periods of undetectable levels between pulses 2, 3
• Avoid sampling during REM sleep, as GH concentrations are significantly lower during REM compared to stage 3 and 4 sleep 2
• Do not use 5-minute or 20-minute sampling intervals for nocturnal studies, as these miss 43% and 64% of GH pulses respectively compared to 1-minute sampling 2
• The 24-hour integrated GH concentration test provides the best diagnostic accuracy, with no overlap between normal controls (lower 5th percentile: 3.3 μg/L) and GH-deficient patients (upper 97th percentile: 2.7 μg/L) 5

Practical Recommendations

For Stimulation Testing

• Use an indwelling needle or soft catheter in an antecubital vein 1
• Draw blood samples from the contralateral arm to avoid contamination with infused arginine 1
• Minimize patient apprehension and distress, particularly in children, as stress affects GH secretion 1

For Spontaneous Secretion Assessment

• The longer the blood collection period, the greater the sensitivity and specificity for detecting GH deficiency 5
• 12-hour nighttime protocols show minimal overlap with normal controls in GH-deficient patients 5
• Abbreviated protocols shorter than 12 hours have considerable overlap with normal values and should be avoided for definitive diagnosis 5