Growth Hormone (GH) is a protein produced by the pituitary gland, playing a fundamental role in stimulating growth and cell reproduction, particularly in children. Because GH is released in short, irregular bursts throughout the day and night, a single blood test cannot accurately measure the gland’s total capacity. The Growth Hormone Stimulation Test (GHST) is a specialized procedure designed to temporarily override this pulsatile release pattern. By administering a substance that forces the pituitary gland to release GH, the test measures the maximum amount of hormone the body can produce, providing a reliable assessment of the pituitary reserve.
Why the Test is Necessary
A physician typically orders the Growth Hormone Stimulation Test when a child shows signs of significantly impaired growth unexplained by other factors. The primary indication is persistently slow growth velocity, meaning the child is falling further behind on standard growth charts. This often accompanies significantly short stature, defined as a height below the third percentile for age and sex.
A child under four years old growing less than six centimeters per year, or an older child growing less than five centimeters annually, is a significant red flag. These growth patterns, combined with other clinical signs, suggest a possible deficiency. Other physical characteristics that may prompt testing include an immature facial appearance, a mildly chubby body build, and a delay in the onset of puberty.
The GHST is a definitive, confirmatory measure used after initial screenings. Before the stimulation test, doctors evaluate the child’s Insulin-like Growth Factor 1 (IGF-1) and IGF-Binding Protein 3 (IGFBP-3) levels, which are stable hormones produced in response to GH. They also assess skeletal maturation using a bone age x-ray. If these initial tests and the clinical picture strongly suggest a deficiency, the GHST confirms the pituitary gland’s ability to secrete the hormone.
Navigating the Testing Process
Preparation for the GHST requires a fasting period of 10 to 12 hours, during which the child may only consume plain water. The procedure is typically conducted in an outpatient clinic and requires two to four hours for the entire process.
Upon arrival, a nurse places a thin intravenous (IV) catheter, usually in the hand or arm. This single IV line is used for the duration of the test, eliminating the need for repeated needle sticks for blood draws. The process begins with a baseline blood sample taken through the IV before any medication is administered.
The physician administers stimulating agents to provoke the pituitary gland into releasing its maximum store of Growth Hormone. These agents may be given intravenously, orally, or by injection, and can include substances such as:
- Arginine
- Clonidine
- Glucagon
Blood samples are drawn at precise, timed intervals, usually every 15 to 30 minutes, for the next few hours. The child must remain relatively calm and still, ideally lying down, because physical activity or emotional stress can artificially increase GH levels. Parents are encouraged to bring quiet activities to occupy the child during the extended testing period.
Decoding the Peak GH Level
The primary measurement derived from the Growth Hormone Stimulation Test is the peak GH level, which is the highest concentration of the hormone recorded in any timed blood sample. This value indicates the maximum amount of GH the pituitary gland released in response to the pharmacological stimulus. A normal response is defined by a peak GH level that reaches or exceeds 10 nanograms per milliliter (ng/mL) of blood.
A result below this threshold suggests a deficiency, but interpretation is nuanced. Peak levels between 5 ng/mL and 10 ng/mL are considered an indeterminate or partial deficiency. A severe deficiency is diagnosed when the peak GH level is below 5 ng/mL.
Interpretation must be made in the context of the child’s overall clinical presentation and other lab results. If IGF-1 and IGFBP-3 levels are also low, it reinforces the diagnosis of a true GH deficiency. Body mass index (BMI) is also a factor, as being overweight or obese can lower the measured peak GH response, potentially leading to an over-diagnosis.
Pubertal status is a consideration, as sex hormones naturally increase GH secretion. Some protocols recommend “priming” prepubertal children with a short course of sex hormones before the test to ensure a maximal response. The GHST result is a single data point that a pediatric endocrinologist integrates with growth charts, bone age, and supporting blood work to arrive at a comprehensive diagnosis.
Follow-Up and Treatment Options
Once Growth Hormone Deficiency (GHD) is confirmed by a low peak GH level and supporting clinical evidence, the standard course of action is Growth Hormone Replacement Therapy (GHRT). This treatment involves administering a synthetic form of human Growth Hormone, known as somatropin, via a daily subcutaneous injection.
The goal of GHRT is to return the child to a normal growth curve, allowing them to achieve an expected final adult height. The dose is calculated based on the child’s weight and adjusted over time. Treatment continues until the child’s growth plates fuse, which occurs in the late teens, signifying the end of linear growth.
Regular monitoring assesses both effectiveness and safety. Physicians track the child’s height velocity and bone age every three to six months to ensure a positive growth response. Laboratory tests, particularly IGF-1 levels, are monitored to keep the hormone concentration within the age-appropriate normal range and prevent side effects.
If the GHST result is normal, showing a peak GH level above the diagnostic threshold, it rules out a classic GHD. In this case, the physician investigates other potential causes for the child’s short stature.
Other Potential Causes
- Celiac disease
- Thyroid disorders
- Familial short stature
A normal test result redirects the diagnostic focus, ensuring the child receives an accurate diagnosis and the most appropriate treatment for their specific condition.