Drugs That Distort Your ACTH Test Results

What ACTH Is and Why Accuracy Matters

Adrenocorticotropic hormone (ACTH) is a 39-amino-acid peptide released from the anterior pituitary in a pulsatile, circadian pattern, peaking between 6 and 9 AM and reaching its nadir near midnight. Its primary job is signaling the adrenal cortex to produce cortisol. A single misleading ACTH value can send a clinician down a months-long workup for Cushing syndrome or adrenal insufficiency that never existed.

The hypothalamic-pituitary-adrenal (HPA) axis operates through tight negative feedback. Cortisol rises, ACTH falls. Anything that mimics cortisol, blocks its synthesis, or damages the pituitary will shift ACTH levels independent of true disease. The 2016 Endocrine Society Clinical Practice Guideline on Cushing syndrome specifically warns that "exogenous glucocorticoid exposure is the most common cause of Cushing syndrome and must be excluded before biochemical testing" [1]. That single sentence explains why a medication history is the first step in any ACTH interpretation.

ACTH also degrades faster than most peptide hormones. Samples left at room temperature lose 20% of measurable ACTH within one hour [2]. This pre-analytical fragility means that drug interference and handling errors can compound, turning a routine lab draw into an uninterpretable result. The Endocrine Society recommends collecting blood into pre-chilled EDTA-containing tubes and separating plasma on ice within 15 minutes of the draw [1].

Exogenous Glucocorticoids: The Most Common Distortion

Any form of exogenous glucocorticoid, whether oral, injectable, inhaled, topical, or intra-articular, can suppress ACTH by substituting for endogenous cortisol in the negative feedback loop. This is the single most frequent cause of a misleadingly low ACTH result in clinical practice.

Oral prednisone at doses as low as 5 mg daily for two weeks can begin suppressing morning ACTH below the reference range [3]. Higher doses act faster. A 2020 meta-analysis in the Journal of Clinical Endocrinology and Metabolism found that 48% of patients on chronic oral glucocorticoids (defined as more than 4 weeks of prednisone-equivalent doses of 7.5 mg/day or higher) had morning cortisol values below 5 mcg/dL, a marker of clinically significant HPA axis suppression that corresponds to suppressed ACTH [4].

Inhaled corticosteroids are easy to overlook. High-dose fluticasone propionate (1 to 000 mcg/day or more) suppresses the HPA axis in roughly 50% of adult users according to data from a systematic review published in the Annals of Internal Medicine [5]. Budesonide carries a somewhat lower systemic absorption profile, but doses above 800 mcg/day still warrant caution.

Topical and intra-articular steroids are the most frequently missed offenders. A single triamcinolone acetonide joint injection (40 to 80 mg) can suppress morning cortisol and ACTH for 2 to 4 weeks [6]. Dermatologists applying clobetasol propionate 0.05% to large body surface areas have documented frank adrenal suppression confirmed by cosyntropin stimulation testing.

The clinical takeaway: ask every patient about steroid use in all forms before interpreting an ACTH result. If glucocorticoid exposure is confirmed, allow a washout period. The Endocrine Society suggests waiting at least 18 to 24 hours after the last dose of a short-acting glucocorticoid, or longer for depot injections, before drawing ACTH [1].

Opioids and Central ACTH Suppression

Opioids suppress ACTH through direct inhibition of corticotropin-releasing hormone (CRH) neurons in the hypothalamus. This is not a rare pharmacologic curiosity. It affects a large percentage of chronic opioid users.

A landmark 2015 cross-sectional study published in the Journal of Clinical Endocrinology and Metabolism examined 109 men on long-term opioid therapy and found that 24% had morning cortisol levels below 5 mcg/dL with correspondingly suppressed ACTH [7]. The effect was dose-dependent. Patients taking morphine-equivalent doses above 60 mg/day were significantly more likely to develop opioid-induced adrenal insufficiency (OIAI) than those on lower doses.

Dr. Maria Fleseriu, an endocrinologist at Oregon Health and Science University, has written that "opioid-induced adrenal insufficiency is underdiagnosed because clinicians do not routinely screen for it, despite its prevalence in chronic pain populations" [8]. Methadone, fentanyl, hydromorphone, and oxycodone all carry this effect. Buprenorphine, a partial agonist, appears to cause less HPA suppression, though data remain limited [7].

For patients on chronic opioids who need ACTH testing, a cosyntropin stimulation test may provide more reliable information about adrenal reserve than a single morning ACTH draw, since basal ACTH in these patients is suppressed by the opioid itself rather than by pituitary or adrenal pathology.

Estrogen, Oral Contraceptives, and CBG Effects

Estrogen-containing oral contraceptives do not directly alter ACTH secretion, but they raise cortisol-binding globulin (CBG) levels by 50% to 100% [9]. This increase in CBG elevates total serum cortisol while leaving free (biologically active) cortisol relatively unchanged. The clinical problem: when total cortisol runs high, the pituitary may modestly adjust ACTH downward, or the clinician may misinterpret the cortisol-ACTH pair as evidence of autonomous cortisol secretion.

A study in the European Journal of Endocrinology found that women on combined oral contraceptives had mean total cortisol levels of 28.3 mcg/dL compared to 14.6 mcg/dL in controls, with ACTH values that were statistically indistinguishable between groups [10]. The mismatch between a high cortisol and a "normal" ACTH can look like mild autonomous cortisol secretion to an uninformed reader.

The fix is straightforward. Discontinue estrogen-containing oral contraceptives for 6 weeks before performing a Cushing syndrome workup. The 2008 Endocrine Society guideline on Cushing syndrome diagnosis explicitly states this recommendation [1]. If discontinuation is not feasible, measuring late-night salivary cortisol or urinary free cortisol (which reflects free rather than bound hormone) provides a workaround.

Hormone replacement therapy with conjugated estrogens or transdermal estradiol also affects CBG, though transdermal formulations bypass first-pass hepatic metabolism and have a smaller impact on binding protein synthesis. Tamoxifen and raloxifene, selective estrogen receptor modulators, can produce similar though less pronounced CBG elevations [9].

Drugs That Raise ACTH by Blocking Cortisol Synthesis

Several medications inhibit enzymes in the adrenal steroidogenesis pathway, lowering cortisol production and triggering a compensatory rise in ACTH through loss of negative feedback.

Metyrapone blocks 11-beta-hydroxylase, the enzyme catalyzing the final step of cortisol synthesis. It is used therapeutically in the metyrapone stimulation test and in the treatment of Cushing syndrome. During a metyrapone test, ACTH levels are expected to rise above 200 pg/mL in patients with intact pituitary function [11]. Outside of testing contexts, metyrapone prescribed for Cushing syndrome will raise ACTH and can drive adrenal androgen overproduction.

Ketoconazole at antifungal doses (200 to 400 mg/day) has modest effects on steroidogenesis. At higher doses used off-label for Cushing syndrome (600 to 1 to 200 mg/day), it inhibits multiple cytochrome P450 enzymes (CYP11A1, CYP17A1, CYP11B1) and reliably lowers cortisol with a corresponding ACTH increase [12]. The FDA approved a related compound, levoketoconazole (Recorlev), in 2021 specifically for endogenous Cushing syndrome, and ACTH elevation is an expected pharmacologic consequence [13].

Etomidate, the intravenous anesthetic, inhibits 11-beta-hydroxylase at sub-hypnotic doses. Even a single induction dose (0.3 mg/kg) suppresses cortisol synthesis for 4 to 8 hours [14]. In critically ill patients receiving etomidate, ACTH levels may rise acutely while cortisol remains paradoxically low, a pattern that can confuse sepsis-related adrenal assessments in the ICU.

Mitotane (Lysodren), used for adrenocortical carcinoma, destroys adrenal cortex tissue and blocks steroidogenesis. Patients on mitotane will have elevated ACTH from loss of cortisol feedback and require lifelong glucocorticoid replacement [15].

Checkpoint Inhibitors and Pituitary Destruction

Immune checkpoint inhibitors (ICIs), including ipilimumab (anti-CTLA-4), nivolumab and pembrolizumab (anti-PD-1), and atezolizumab (anti-PD-L1), can trigger autoimmune hypophysitis that destroys corticotroph cells. The result is a low ACTH with low cortisol, mimicking secondary adrenal insufficiency.

Hypophysitis occurs in 5% to 17% of patients treated with ipilimumab, and in 0.5% to 1.5% of those on PD-1 inhibitors [16]. The rate climbs higher with combination regimens. A retrospective analysis of 496 patients on ipilimumab plus nivolumab at Memorial Sloan Kettering found an 8.8% incidence of hypophysitis requiring hormone replacement [17]. ACTH deficiency was the most common pituitary hormone deficit, present in over 90% of confirmed cases.

The 2021 ASCO Clinical Practice Guideline on immune-related adverse events recommends "baseline morning cortisol and ACTH before initiating CTLA-4 therapy, with repeat testing at each cycle for the first 12 weeks" [18]. Symptoms of ICI-related hypophysitis (fatigue, headache, nausea, hypotension) overlap with common cancer treatment side effects, making the biochemical surveillance critical.

Unlike glucocorticoid-induced ACTH suppression, ICI-related ACTH deficiency is usually permanent. The pituitary corticotroph cells do not regenerate after immune-mediated destruction. Patients require lifelong hydrocortisone replacement, typically 15 to 25 mg daily in divided doses [16].

Psychiatric Medications, Anticonvulsants, and Other Disruptors

Several drug classes outside the usual endocrine suspects can shift ACTH values enough to complicate interpretation.

Antipsychotics and SSRIs. Serotonin stimulates CRH release, and SSRIs can transiently raise ACTH and cortisol during the first weeks of therapy. A study in Psychoneuroendocrinology showed that citalopram 20 mg/day increased ACTH by 36% and cortisol by 27% in the first two weeks, with values normalizing by week eight [19]. Atypical antipsychotics, particularly olanzapine and clozapine, have been associated with HPA axis dysregulation, though the clinical significance for ACTH testing is less well-defined.

Anticonvulsants. Phenytoin, carbamazepine, and phenobarbital induce hepatic CYP3A4, which accelerates cortisol metabolism. The resulting drop in circulating cortisol can trigger a compensatory ACTH increase [20]. This enzyme induction effect is relevant for patients undergoing dexamethasone suppression testing, as these drugs also accelerate dexamethasone clearance, potentially causing false-positive results.

Megestrol acetate. This synthetic progestin, commonly used as an appetite stimulant in oncology and geriatric medicine, has intrinsic glucocorticoid receptor activity. Chronic use can suppress the HPA axis identically to exogenous glucocorticoids. A review in the Annals of Pharmacotherapy documented adrenal insufficiency in patients on megestrol acetate 400 to 800 mg/day, with ACTH suppression mirroring that seen with prednisone [21].

Mifepristone (Korlym), a glucocorticoid receptor antagonist approved for hyperglycemia in Cushing syndrome, blocks cortisol action at the receptor level without reducing cortisol production. ACTH and cortisol both rise, sometimes dramatically, because the negative feedback signal never reaches the pituitary despite high circulating cortisol [22]. An ACTH drawn during mifepristone therapy is uninterpretable in isolation.

How to Time ACTH Testing Around Interfering Medications

The most reliable approach involves three steps: identify all medications with HPA axis activity, hold or account for them when clinically safe, and document any drugs that cannot be discontinued on the lab requisition.

For short-acting oral glucocorticoids (prednisone, prednisolone, hydrocortisone), hold the dose for 18 to 24 hours before a morning ACTH draw [1]. For depot injections of triamcinolone or methylprednisolone, wait 4 to 6 weeks. For inhaled or topical steroids at high doses, consider a 2-week washout if feasible.

Opioids cannot always be safely discontinued. In chronic pain patients on stable opioid therapy, a cosyntropin stimulation test (250 mcg IV with cortisol measured at 0 and 30 to 60 minutes) provides more actionable data than a basal ACTH alone [7]. A peak cortisol above 18 mcg/dL at 30 minutes generally excludes clinically significant adrenal insufficiency regardless of the basal ACTH value.

For patients on estrogen-containing contraceptives or HRT, the 6-week washout before Cushing testing is standard [1]. If the clinical question is adrenal insufficiency rather than Cushing syndrome, salivary cortisol testing at 8 AM avoids the CBG confound entirely.

For checkpoint inhibitor patients, no washout is possible or appropriate. Instead, track serial ACTH levels. A declining trend in morning ACTH values over successive treatment cycles, particularly dropping below 10 pg/mL, should trigger a cosyntropin stimulation test and endocrinology consultation [18].

Always draw ACTH between 7 and 9 AM, fasting, in a pre-chilled EDTA tube processed on ice. Document the exact time of draw, the last dose and timing of any interfering medication, and the patient's position (upright vs. supine, as posture can shift ACTH by 10% to 20%). A result without this context is a number without meaning.