A stim test, short for stimulation test, is a diagnostic procedure where a doctor injects a substance that triggers a specific gland to produce hormones, then measures your blood levels over the next 30 to 90 minutes to see how well that gland responds. The most common version is the ACTH stimulation test, which checks whether your adrenal glands can produce enough cortisol. Other types evaluate growth hormone production or monitor thyroid cancer after treatment.
How a Stimulation Test Works
The basic principle behind every stim test is the same: give the body a signal it should respond to, then measure the response. Under normal conditions, your glands communicate through a chain of chemical signals. The pituitary gland in your brain sends hormones to other glands (adrenals, thyroid, others), telling them to ramp up production. A stim test mimics or amplifies that signal with an injection, then checks whether the target gland responds the way it should.
If the gland produces an appropriate amount of hormone after being stimulated, it’s functioning normally. If the response is weak or absent, that points to a problem with either the gland itself or the signaling pathway from the brain.
The ACTH Stimulation Test
This is by far the most common stim test. It evaluates adrenal function by injecting a synthetic version of ACTH, the hormone your pituitary normally sends to your adrenal glands to trigger cortisol production. Cortisol plays a central role in your body’s stress response, blood sugar regulation, and immune function, so insufficient production (adrenal insufficiency) can cause fatigue, low blood pressure, weight loss, and other serious symptoms.
During the test, a nurse or technician places an IV line and draws a baseline blood sample. You then receive an injection of the synthetic ACTH. Blood is drawn again at 30 minutes and 60 minutes. The entire appointment typically lasts about 90 minutes. A normal result is a cortisol level that rises above 12.6 micrograms per deciliter after the injection. Anything below that threshold suggests the adrenal glands aren’t responding properly.
Standard Dose vs. Low Dose
The standard test uses a 250-microgram dose, which is actually much higher than the amount of ACTH your body produces naturally. This large dose is reliable for catching moderate to severe adrenal insufficiency, but it can sometimes overwhelm a mildly underperforming adrenal gland into producing enough cortisol to appear normal, potentially missing early or partial cases.
A low-dose version uses just 1 microgram, which is closer to what the body naturally produces. Blood is drawn at 20 and 30 minutes instead. This version is considered as sensitive or even more sensitive than the standard test for detecting early stages of adrenal insufficiency. In practice, many endocrinologists use the standard dose first and reserve the low-dose test for situations where results are borderline or mild adrenal problems are suspected.
Growth Hormone Stimulation Test
This stim test checks whether the pituitary gland can produce adequate growth hormone. It’s commonly ordered for children who are growing significantly slower than expected, and for adults with pituitary damage from surgery, radiation, or a tumor. Unlike the ACTH test, the growth hormone version works by triggering a stress response, most often through insulin-induced low blood sugar.
You receive insulin through an IV, and once your blood sugar drops below 40 mg/dL and you start feeling symptoms like sweating and shakiness, your body should respond by releasing growth hormone. Blood samples are collected at multiple intervals over about 90 minutes. A peak growth hormone level of 5 micrograms per liter or lower confirms growth hormone deficiency in adults. Because of the intentional drop in blood sugar, this test is done under close medical supervision with glucose readily available.
TSH Stimulation Test for Thyroid Cancer
This version serves a very different purpose. Rather than diagnosing a hormonal deficiency, it’s used to monitor people who have already been treated for thyroid cancer. After thyroid removal, doctors track a protein called thyroglobulin, which is produced only by thyroid cells. If thyroglobulin is still detectable in your blood, it could mean some thyroid cancer cells remain.
The problem is that thyroglobulin levels can be too low to detect through routine blood work, even when small amounts of cancer persist. A TSH stimulation test solves this by injecting synthetic TSH on day one and day two, which “wakes up” any remaining thyroid cells and stimulates them to produce thyroglobulin. On day five, a blood draw measures whether thyroglobulin levels have risen. A significant increase signals that thyroid cells are still present and further treatment may be needed.
What to Expect During the Procedure
Most stim tests follow a predictable routine. You’ll typically need to fast beforehand, and your doctor may ask you to temporarily stop certain medications that affect hormone levels, particularly steroids. The test is done in an outpatient setting, usually a hospital lab or endocrinology clinic.
After check-in, an IV catheter is placed in your arm for blood draws and the injection. The waiting between blood draws is the longest part. For an ACTH test, you’ll be done in about an hour. Growth hormone testing can take closer to two hours because of the additional monitoring involved. You’ll sit or lie down in the clinic during the waiting periods.
Side effects from the synthetic ACTH used in adrenal testing are uncommon but can include flushing, rash, or a temporary change in heart rate. Allergic reactions are rare, and the test is not performed on anyone with a known allergy to the synthetic hormone. The growth hormone test carries more noticeable short-term discomfort because of the intentional blood sugar drop, but the medical team monitors you throughout and will give you glucose if needed.
Understanding Your Results
Results from a stim test are usually straightforward. Your doctor compares your peak hormone level to an established threshold. For the ACTH stimulation test, a cortisol level above 12.6 mcg/dL is considered a normal response. For growth hormone, a peak above 5 mcg/L rules out deficiency in adults.
A failed stim test doesn’t always mean you need lifelong treatment. Context matters. If you’re being tested because you recently stopped taking steroids, for example, your adrenal glands may just need more time to “wake up” after being suppressed. Current Endocrine Society guidelines note that a simple morning cortisol blood draw above 10 mcg/dL can sometimes confirm recovery without needing a full stimulation test at all. Your endocrinologist will interpret the stim test result alongside your symptoms, medical history, and other lab work to determine next steps.