17-Hydroxyprogesterone (17-OHP) is a steroid hormone precursor produced by the adrenal glands, used to create other hormones. Measuring 17-OHP levels in the blood is a standard medical procedure with a specific diagnostic purpose. The test assesses the function of the adrenal glands, which are small organs located on top of each kidney. An elevated level of this precursor is a clear sign that a particular step in the hormone creation process is blocked or slowed down.
The Biological Role of 17-Hydroxyprogesterone
17-OHP functions as an intermediate molecule in steroidogenesis, the pathway that creates steroid hormones. It is produced primarily in the adrenal glands and, to a lesser extent, in the gonads. This precursor is necessary for the production of cortisol, a hormone that manages the body’s response to stress, regulates metabolism, and supports the immune system.
The conversion of 17-OHP into cortisol is controlled by a sequence of enzymes. The enzyme 21-hydroxylase is responsible for converting 17-OHP into 11-deoxycortisol, a step preceding the final formation of cortisol. If this enzyme functions correctly, 17-OHP is quickly processed and its levels remain low.
17-OHP is also a precursor to sex steroids, such as androgens (male sex hormones). When the pathway toward cortisol is blocked, excess 17-OHP is diverted into other routes. This diversion leads to the overproduction of androgens, which can cause the development of male characteristics in both sexes.
Why 17-OHP Levels Are Tested for Congenital Adrenal Hyperplasia
The primary reason for testing 17-OHP levels is to screen for and diagnose Congenital Adrenal Hyperplasia (CAH). CAH is a group of inherited disorders that impairs the adrenal glands’ ability to produce necessary hormones. This impairment is most commonly due to a deficiency of the 21-hydroxylase enzyme, which accounts for over 90% of all CAH cases.
When 21-hydroxylase is deficient, the conversion of 17-OHP to cortisol is severely impaired or blocked. Since the body senses a lack of cortisol, the pituitary gland releases adrenocorticotropic hormone (ACTH) to stimulate the adrenal glands. This continuous stimulation causes the adrenal glands to enlarge, a condition known as hyperplasia.
This metabolic logjam causes the 17-OHP precursor to accumulate dramatically in the bloodstream. The buildup of 17-OHP is a direct biochemical marker for the most common form of CAH. The 17-OHP test is used as a standard part of newborn screening programs worldwide to detect the severe, or classic, form of CAH.
Classic CAH can manifest as salt-wasting CAH, where the body cannot produce enough aldosterone, a hormone that regulates salt and water balance. The accumulated 17-OHP and progesterone also have an anti-mineralocorticoid effect, which worsens salt loss. Measuring this elevated precursor is a time-sensitive method to identify infants at risk for a life-threatening adrenal crisis.
How the 17-OHP Test is Performed
The procedure for measuring 17-OHP is a blood test, though the exact method depends on the age of the person being tested. For infants, the test is routinely performed as part of newborn screening using a heel stick. A small amount of blood is collected onto a specialized filter paper card and sent to a laboratory for analysis.
The timing of the test is important, especially for newborns. 17-OHP levels are naturally elevated immediately after birth and decrease over the first one to two days of life. To avoid false-positive results, newborn screening is typically performed between 24 and 48 hours after birth. Premature or stressed infants often have higher baseline 17-OHP concentrations, which may necessitate a repeat test to confirm the results.
For older children and adults, the test involves a standard venipuncture, where a blood sample is drawn from a vein in the arm. Factors such as the time of day, the menstrual cycle phase in women, and certain medications can influence the results. For example, corticosteroids can interfere with the test accuracy, so doctors may instruct a temporary pause in medication before the blood draw.
Interpreting Abnormal 17-OHP Results
A significantly high 17-OHP concentration is the primary finding suggesting a diagnosis of CAH. Levels in infants with classic CAH can range dramatically, often exceeding 2,000 nanograms per deciliter (ng/dL), with some severe cases reaching over 10,000 ng/dL. These elevations reflect the severe block in the cortisol production pathway.
Markedly elevated levels necessitate immediate follow-up testing to confirm the diagnosis and assess the condition’s severity. In newborns, a high result raises concern for a salt-wasting crisis, characterized by dehydration, vomiting, and dangerously low sodium levels, requiring prompt medical intervention. The excess androgens produced by the 17-OHP diversion also cause virilization, which can lead to ambiguous genitalia in female infants.
Milder elevations, often in the range of 200 to 800 ng/dL, may indicate nonclassic CAH, or late-onset CAH. This milder form may present later in childhood or adulthood with subtle symptoms such as excess facial hair, acne, or irregular menstrual periods in women. Once CAH is diagnosed, a low or decreasing 17-OHP level in a treated patient indicates that the prescribed hormone replacement therapy, which typically includes cortisol, is working effectively.