18-Hydroxycorticosterone Test

If you have high blood pressure that does not respond well to treatment, or if your potassium keeps running low without a clear reason, there may be a problem hiding in your adrenal glands. 18-hydroxycorticosterone (18-OHB) is one of the few blood tests that can help pinpoint the source, specifically whether one or both adrenal glands are overproducing aldosterone, the hormone that controls your body's salt and water balance.

This is a specialized, Tier 2 diagnostic marker. It is not part of routine blood work and does not have universally standardized clinical cutpoints. But in the right clinical context, particularly when primary aldosteronism has been confirmed, 18-OHB can provide a valuable clue about whether the problem comes from a single adrenal tumor or from both glands working too hard. That distinction determines whether surgery could cure your hypertension.

What 18-OHB Actually Is

Your adrenal glands sit on top of your kidneys and produce several hormones. The outermost layer of the adrenal cortex, called the zona glomerulosa, is responsible for making aldosterone. It does this through a three-step chemical conversion, all performed by a single enzyme called aldosterone synthase. 18-OHB is the product of the second step: the enzyme takes corticosterone (an earlier steroid) and adds a chemical group at the 18th position, creating 18-hydroxycorticosterone. In the third and final step, that same enzyme converts 18-OHB into aldosterone.

Under normal conditions, 18-OHB and aldosterone rise and fall together in a remarkably stable ratio of about 2.5 to 1. Both respond to the same signals: the renin-angiotensin system (your body's main blood pressure regulation pathway), potassium levels, and ACTH (a pituitary hormone that stimulates the adrenal glands). Because 18-OHB tracks aldosterone production so closely, measuring it gives you a window into how hard your adrenal glands are working to produce aldosterone.

Primary Aldosteronism: The Main Reason to Test

Primary aldosteronism (PA) is a condition in which one or both adrenal glands produce too much aldosterone, leading to high blood pressure and often low potassium. It is far more common than most people realize. The condition is the most frequent identifiable, treatable cause of hypertension, and screening guidelines now recommend considering PA in all people with high blood pressure, not just those with severe or resistant cases.

The standard screening test for PA is the aldosterone-to-renin ratio (ARR), which has a sensitivity of approximately 92% when optimized cutoffs are used. 18-OHB is not a screening test. Its value comes after PA has been confirmed, when you need to answer a different question: is the excess aldosterone coming from a single nodule in one adrenal gland (an aldosterone-producing adenoma, or APA), or from both glands being overactive (bilateral adrenal hyperplasia, or BAH)?

Distinguishing Tumor from Bilateral Overgrowth

This distinction matters enormously. If a single tumor is the source, surgically removing that adrenal gland can cure the hypertension entirely. If both glands are overactive, surgery will not help, and the treatment is lifelong medication with a mineralocorticoid receptor blocker like spironolactone. Making the wrong call means either unnecessary surgery or a missed chance at a cure.

Classically, people with an aldosterone-producing adenoma have morning recumbent (lying-down) plasma 18-OHB levels above 100 ng/dL, while those with bilateral hyperplasia typically remain below that threshold. In early studies, this cutpoint showed 100% specificity for identifying adenomas. However, clinical guidelines now caution that 18-OHB alone should not determine whether surgery is performed. More than 40% of patients cannot be reliably classified by steroid measurements alone, and adrenal vein sampling remains the gold standard for confirming which gland is the source.

Other Conditions That Shift 18-OHB

Beyond primary aldosteronism subtyping, 18-OHB levels help identify several rarer adrenal conditions. In familial hyperaldosteronism type 1 (also called glucocorticoid-remediable aldosteronism), a genetic condition in which aldosterone production comes under the control of ACTH rather than the renin-angiotensin system, 18-OHB levels are among the highest seen in any condition. Familial hyperaldosteronism type 3 also produces very high levels.

Certain forms of congenital adrenal hyperplasia (inherited enzyme deficiencies that disrupt steroid production) produce distinctive 18-OHB patterns. In 17-alpha-hydroxylase deficiency, 18-OHB is dramatically elevated with an abnormally high ratio of 18-OHB to aldosterone (above 230). In 21-hydroxylase deficiency, both 18-OHB and aldosterone are elevated. In 11-beta-hydroxylase deficiency, 18-OHB is reduced along with aldosterone. These patterns can help endocrinologists distinguish between enzyme deficiencies when clinical features overlap.

Low or undetectable 18-OHB occurs in Addison's disease (adrenal insufficiency, where the adrenal glands are damaged and underproduce hormones) and in corticosterone methyloxidase deficiency (a rare inherited defect in the final conversion steps of aldosterone production).

Diagnostic Performance in Context

When 18-OHB is combined with related steroids (18-hydroxycortisol, 18-oxocortisol, tetrahydroaldosterone, and aldosterone) in a urinary multi-steroid panel measured by mass spectrometry, one study found a combined sensitivity for PA diagnosis of about 87% with specificity of 88%. A separate large multicenter study using machine learning with steroid profiling showed 69% sensitivity and 94% specificity for detecting PA. These numbers are lower than the ARR alone for general screening, but steroid profiling appears to perform better for identifying specific genetic subtypes, particularly mutations in the KCNJ5 gene, which tend to predict higher surgical cure rates.

18-OHB has very low biological activity on its own. It does not act as a meaningful mineralocorticoid in your body the way aldosterone does. Its value is purely as a diagnostic signal, reflecting how much aldosterone synthase activity is occurring in your adrenal glands.

Reference Ranges

Reference ranges for 18-OHB are not standardized across laboratories, and results depend heavily on the assay method used. Older radioimmunoassay studies reported normal values as mean levels of about 10.3 ng/dL in males and 12.4 to 13.8 ng/dL in females. More recent measurements using a specialized lab technique called liquid chromatography-tandem mass spectrometry (LC-MS/MS) in 691 healthy individuals found a combined reference interval of approximately 90.5 to 1,040.6 pg/mL (equivalent to roughly 9 to 104 ng/dL), with higher levels in females than males.

These values are drawn from published research and vary by assay method, lab, and collection conditions. The 100 ng/dL cutpoint applies specifically to subtyping primary aldosteronism in patients already diagnosed with the condition, not to screening healthy people. Always compare your results within the same lab over time rather than treating any single threshold as absolute.

When Results Can Be Misleading

18-OHB has a strong circadian rhythm, peaking in the early morning alongside ACTH and falling throughout the day. Drawing blood at the wrong time, or after the patient has been upright and active rather than lying down, can produce significantly different numbers. The standard protocol calls for an early morning blood draw (around 8:00 AM) after the patient has been recumbent overnight. Upright posture alone can raise 18-OHB substantially by activating the renin-angiotensin system.

Dietary sodium intake is one of the strongest influences on 18-OHB. In healthy men, switching from a high-sodium diet (200 mmol/day) to a low-sodium diet (40 mmol/day) produced a roughly 6-fold increase in 18-OHB, from about 9.8 ng/dL to 62.5 ng/dL. If your sodium intake is unusually low or high in the days before testing, your result may not reflect your baseline adrenal function.

Sex and age both affect levels. Females tend to have higher 18-OHB than males, and levels generally decline with advancing age, consistent with the general decline in adrenal steroid production over time. Potassium levels also influence production directly: high potassium stimulates the zona glomerulosa, raising both aldosterone and 18-OHB.

Because 18-OHB requires specialized LC-MS/MS assays not available at most hospitals, results from different laboratories may not be directly comparable. Different assay methods can give meaningfully different numbers for the same blood sample.

Tracking Your Trend

A single 18-OHB reading is a snapshot taken under specific conditions (time of day, body position, sodium intake, potassium status). Because so many factors influence the number, repeating the test under the same standardized conditions gives you far more confidence in the result. If your first reading suggests an adenoma and you are considering surgery, confirming with a second measurement (and ideally adrenal vein sampling) is the standard of care.

For people being treated medically for bilateral adrenal hyperplasia, periodic retesting can help confirm that the underlying adrenal overactivity is stable. If levels begin climbing, it may signal progression or the development of a nodule that warrants further imaging. A baseline test, a confirmatory retest under identical collection conditions, and then follow-up at least annually (or sooner if your clinical picture changes) is a reasonable cadence.

Recent research shows that steroid profiling accuracy improves significantly when measurements are taken after ACTH stimulation rather than at baseline alone, with the area under the curve for subtype differentiation reaching 0.957 after stimulation. If your initial baseline result is ambiguous, an ACTH-stimulated measurement may provide a clearer answer.