A growth hormone (GH) stimulation test is a medical procedure that assesses the pituitary gland’s ability to produce and release growth hormone. This small gland, located near the center of the head, is responsible for secreting GH, which plays a role in growth, body composition, and metabolism. It prompts the pituitary gland to release GH and measures its response to determine if it functions as expected.
Reasons for the Test
A doctor may order a growth hormone stimulation test when Growth Hormone Deficiency (GHD) is suspected. Symptoms vary between children and adults. In children, indications often include a slow growth rate, such as growing less than 1.4 inches per year after age three. Other signs include appearing younger than their age, impaired hair and nail growth, delayed tooth development, or delayed puberty. Infants and toddlers might have low blood sugar.
Adults with GHD experience different symptoms. These may include persistent fatigue, loss of muscle mass, and increased body fat, especially around the waist. They may also report reduced strength, bone weakness, or psychological symptoms like depression.
The Testing Procedure
Preparation is required for accurate results. Patients fast for 10 to 12 hours before the test; only plain tap water is permitted. Young children may have a shorter fasting period, around four hours, which should be discussed with the medical team. Strenuous exercise should be avoided for at least 24 hours prior.
Upon arrival at the clinic, a healthcare professional inserts an intravenous (IV) line, typically in an arm or hand vein. A baseline blood sample is collected through the IV to establish the starting GH level. Following this, a “stimulating agent” is administered, either through the IV or orally, to prompt the pituitary gland to release growth hormone.
Stimulating agents include arginine, clonidine, glucagon, or insulin. The insulin tolerance test (ITT) is sometimes used for adult GHD diagnosis, but its use may be limited due to safety. After the agent is given, blood samples are drawn from the IV line at timed intervals, typically every 30 minutes, over two to five hours. Patients remain resting during this time, as some medications can cause lightheadedness or dizziness.
Understanding the Results
After the blood samples are collected during the test, they are sent to a laboratory for analysis to determine the “peak” growth hormone level achieved. The lab examines the series of samples to identify the highest GH concentration reached in response to the stimulating agent. A “normal” response indicates that the pituitary gland is functioning as expected, with the GH level rising above a certain threshold.
For pediatric patients, a peak GH value of at least 10 ng/mL (10 µg/L) is often considered normal, though some laboratories may use a cutoff of 7 ng/mL (7 µg/L). Conversely, a “subnormal” or “failing” response, typically defined as a peak GH level below 5 ng/mL (5 µg/L), suggests Growth Hormone Deficiency. Values between 5 and 10 ng/mL may be classified as indeterminate, requiring further evaluation. The specific cutoff values can differ based on the laboratory conducting the test and the stimulating agent used, along with individual patient factors such as age and body mass index. These results are always interpreted in conjunction with other clinical factors and additional blood tests, like insulin-like growth factor-1 (IGF-1) levels, to confirm a diagnosis.
Potential Side Effects and Post-Test Care
Patients undergoing a growth hormone stimulation test may experience various side effects, which are closely monitored by medical staff throughout the procedure. The specific reactions depend on the stimulating agent administered. For instance, insulin can cause hypoglycemia, or low blood sugar, which is carefully managed to prevent complications. Clonidine, another agent, might lead to drowsiness, dizziness, low blood pressure, dry mouth, or blurry vision.
Glucagon, when used as a stimulant, can also cause a drop in blood sugar, along with nausea, vomiting, or abdominal pain. Arginine may induce lightheadedness, nausea, or irritation at the IV insertion site. Medical professionals continuously check blood glucose levels, particularly with insulin and glucagon, to ensure patient safety.
After the test is completed and the patient’s condition is stable, such as when blood sugar levels have normalized and they have had a snack or meal, the IV line is removed. Patients are generally able to go home following the procedure. However, it is recommended to drink plenty of fluids for the rest of the day and plan for rest, avoiding strenuous activities or driving, as some lingering side effects like sleepiness can persist for several hours.