Diagnosing growth hormone deficiency (GHD) in adults requires a combination of clinical suspicion, blood work, and in most cases, a specialized stimulation test that measures how your pituitary gland responds to a deliberate challenge. Unlike many hormone problems, a simple blood draw isn’t enough to confirm or rule out the diagnosis. The process typically starts with your doctor recognizing a pattern of symptoms and risk factors, then moves through increasingly specific testing.
Why a Single Blood Test Isn’t Enough
The first thing most people expect is a straightforward lab result, but growth hormone doesn’t work that way. Your body releases GH in pulses throughout the day, with levels dropping to near zero between pulses. A random blood draw could catch you at a peak or a trough, making it useless for diagnosis.
Instead, doctors rely on a related marker called IGF-1 (insulin-like growth factor 1), which reflects your overall GH activity more steadily. If your IGF-1 level falls more than two standard deviations below the age-matched average, that’s a strong signal pointing toward GHD, especially if you already have known pituitary disease. But here’s the catch: between 37% and 70% of adults with confirmed GH deficiency still have a normal IGF-1 level. The percentage is even higher in people over 40, where only about 35% of GH-deficient adults show an abnormally low IGF-1. So a normal result does not rule out the diagnosis. It simply means the next step, a stimulation test, becomes essential.
Who Should Be Tested
Doctors don’t test everyone with fatigue or weight gain. GHD in adults almost always traces back to a specific cause involving the pituitary gland or the hypothalamus, the brain region that controls it. The most common reasons to suspect GHD include:
- Pituitary tumors or their treatment. Surgery or radiation to the pituitary area is the leading cause of adult GHD.
- Traumatic brain injury. Pituitary damage after head trauma is frequent yet often overlooked. Studies confirm a high prevalence of GHD in people with chronic TBI, and undiagnosed deficiency can slow rehabilitation and recovery.
- Childhood-onset GHD. Adults who were treated for GH deficiency as children need retesting, since some will recover normal function and others won’t.
- Other pituitary hormone deficiencies. If you’ve already been diagnosed with deficiencies in other pituitary hormones (thyroid-stimulating hormone, cortisol, sex hormones), the likelihood of GHD rises sharply with each additional deficiency.
- Structural abnormalities. Congenital malformations of the pituitary or hypothalamus, or conditions like empty sella syndrome, raise suspicion.
Recognizing the Clinical Picture
The symptoms of adult GHD are real but nonspecific, which is part of what makes diagnosis tricky. They overlap with aging, depression, and thyroid problems. The characteristic pattern includes increased body fat concentrated around the midsection, reduced muscle mass and strength, decreased exercise tolerance, and low energy. Many people also notice changes in mood and a general decline in well-being.
Beyond what you feel, GHD causes measurable metabolic changes: elevated total and LDL cholesterol, reduced HDL cholesterol, and decreased insulin sensitivity that can progress to impaired glucose tolerance. Bone mineral density drops, increasing fracture risk. Some people experience reduced sweating and poor temperature regulation. Cardiac function can decline subtly over time. None of these findings alone point to GHD, but when several cluster together in someone with a known pituitary issue, they build a case for formal testing.
The Insulin Tolerance Test
The insulin tolerance test (ITT) has long been considered the gold standard for diagnosing adult GHD. During the test, you receive an injection of insulin that temporarily drops your blood sugar low enough to stress the body. That stress is supposed to trigger a surge of growth hormone from the pituitary. A doctor then measures the peak GH level in your blood over the following 60 to 90 minutes.
If your peak GH stays below 3 to 5 micrograms per liter, the test confirms deficiency. Different guidelines use slightly different thresholds. The Growth Research Society and the Endocrine Society generally use a cutoff below 3 mcg/L, while the American Association of Clinical Endocrinologists uses 5 mcg/L or less.
The ITT is effective but not comfortable. You’ll feel the symptoms of low blood sugar: sweating, shakiness, hunger, and sometimes anxiety. It requires close medical supervision with a physician present throughout. More importantly, the test is off-limits for certain people. If you have a history of seizures, coronary artery disease, or are pregnant, the ITT is contraindicated. It’s also generally avoided in adults over 55.
The Macimorelin Oral Test
For people who can’t safely undergo the ITT, or for clinics that prefer a simpler option, the macimorelin test offers a well-validated alternative. Instead of an insulin injection, you drink a liquid solution that stimulates GH release through a different mechanism. Blood samples are then drawn over roughly 90 minutes to measure your peak GH response.
The recommended cutoff for this test is 5.1 mcg/L. At that threshold, macimorelin delivers 92% sensitivity and 96% specificity, meaning it catches the vast majority of true cases while rarely producing a false positive. A study published in the Journal of Clinical Endocrinology & Metabolism found 94% agreement between macimorelin and the ITT when both used the same 5.1 mcg/L cutoff. The test is well tolerated, reproducible, and doesn’t require the intensive monitoring that the ITT demands.
The Glucagon Stimulation Test
The glucagon stimulation test (GST) is another alternative, particularly useful when both the ITT and macimorelin are unavailable. An injection of glucagon triggers a GH response over two to three hours. The standard diagnostic cutoff is a peak GH below 3 mcg/L.
However, this cutoff has a significant limitation in people who are overweight or obese. Body fat suppresses GH secretion independently of any pituitary problem, which means heavier individuals produce less GH on any stimulation test. Research has shown that using the standard 3 mcg/L cutoff on the glucagon test would misclassify 45% of healthy overweight controls as GH deficient. In overweight and obese men, a much lower cutoff of about 1 mcg/L provided 90% sensitivity and 94% specificity. If you have a higher BMI, your endocrinologist should interpret the glucagon test with adjusted expectations, though no universally accepted BMI-specific cutoff has been formally established across all guidelines.
A Test That’s No Longer Available
You may encounter references to the GHRH-arginine test, which was once a popular and reliable alternative to the ITT. This test combined a synthetic version of the brain hormone that triggers GH release with an amino acid infusion. It worked well and had BMI-adjusted cutoffs that made it useful for a wide range of patients. Unfortunately, recombinant GHRH has been unavailable in the United States since 2008, and global availability has declined to the point where the test is no longer feasible in most clinical settings.
When Stimulation Testing Can Be Skipped
In a narrow set of circumstances, doctors can diagnose GHD without putting you through a stimulation test at all. If you have deficiencies in three or more other pituitary hormones, a low IGF-1 level (at least two standard deviations below normal), and a known structural or genetic condition affecting the pituitary or hypothalamus, the diagnosis is considered established. This shortcut reflects the reality that when the pituitary is severely damaged, the chance of GH being the one hormone still functioning normally is extremely small.
What the Diagnostic Process Looks Like in Practice
For most adults, the path follows a predictable sequence. Your endocrinologist reviews your medical history for pituitary risk factors and evaluates your symptoms and body composition. They order baseline blood work including IGF-1 and tests for other pituitary hormones. If suspicion remains after that initial workup, you’re scheduled for a stimulation test. Which test you get depends on your health profile: the ITT if you’re relatively young and healthy, macimorelin if it’s available at your center, or the glucagon test as a backup. Your weight and any contraindications guide the choice.
The stimulation test itself is typically done in an outpatient endocrine clinic or hospital unit. You’ll need to fast beforehand, and the entire visit takes two to four hours depending on the test. Results are usually interpreted alongside your IGF-1 level, symptom profile, and the number of other pituitary deficiencies present. A single low test result in someone with a clear pituitary cause and matching symptoms is generally sufficient for diagnosis.