Low levels of angiotensin converting enzyme (ACE) in the blood can stem from a surprisingly wide range of causes, from something as straightforward as a common blood-pressure medication to less obvious factors like thyroid disorders, zinc deficiency, genetic variation, and even pregnancy complications. ACE is an enzyme best known for converting a precursor hormone into angiotensin II, which raises blood pressure, and for breaking down bradykinin, a molecule that lowers it.1PubMed Central. Angiotensin Converting Enzymes Because doctors most often order an ACE blood test when investigating sarcoidosis or monitoring granulomatous diseases, a low reading can be confusing. The reality is that the test reflects much more than one disease, and understanding what pushes ACE down helps avoid misinterpretation.
ACE Inhibitor Medications Are the Most Common Culprit
If you take a blood-pressure drug whose generic name ends in “-pril” (lisinopril, enalapril, ramipril, and others), your serum ACE level will almost certainly be artificially low. These drugs work by blocking the enzyme directly, and that blockade shows up in the lab. A study examining hospital records found that ACE levels measured in patients already receiving ACE inhibitor therapy were substantially lower than expected, and lab assays showed significant inhibition at the drug concentrations people actually reach during treatment.2PubMed. Ordering of the Serum Angiotensin-Converting Enzyme Test in Patients Receiving Angiotensin-Converting Enzyme Inhibitor Therapy: An Avoidable but Common Error This seems obvious once you know it, yet the same research highlighted that clinicians frequently order the ACE test without realizing the patient is already on an ACE inhibitor, leading to misleading results. If your ACE level comes back low and you are on one of these medications, the drug is almost certainly the explanation, and the test result tells your doctor nothing useful about your underlying enzyme activity.
Genetics Set Your Baseline
Even without any medication or illness, people vary widely in how much ACE circulates in their blood. A large part of that variation is genetic. Research in animal models engineered to carry different numbers of copies of the ACE gene showed that serum ACE activity ranged from about 62% of normal in animals with one copy up to 144% of normal in those with three copies.3PubMed. Angiotensin-converting enzyme gene mutations, blood pressures, and cardiovascular homeostasis In people, a well-known variation in the ACE gene (commonly called the insertion/deletion polymorphism) accounts for a large share of the natural spread in ACE levels across a population. Someone who inherits two copies of the “insertion” variant can have an ACE level that looks low compared to the lab’s reference range, yet is perfectly normal for their genotype. This genetic baseline matters because a single ACE measurement, taken in isolation, may not mean what it seems.
Hypothyroidism Drags ACE Down
Thyroid hormones have a direct relationship with ACE levels. When thyroid function drops, so does ACE. A study of hypothyroid patients found their average serum ACE level was lower than the average in people with normal thyroid function, and when those same patients received thyroid hormone replacement, their ACE levels climbed back up.4JAMA. Serum Angiotensin-Converting Enzyme: Alterations in Hyperthyroidism, Hypothyroidism, and Subacute Thyroiditis Separate research in patients with Graves’ disease (an autoimmune condition that can swing between overactive and underactive thyroid states during treatment) confirmed that ACE levels tracked closely with thyroid hormone levels, rising during hyperthyroid phases and falling during hypothyroid phases.5PubMed. Thyroid hormones affect serum angiotensin I converting enzyme levels The take-home point: interpreting an ACE blood test without knowing the patient’s thyroid status can lead a doctor astray, because a low ACE level might simply reflect an underactive thyroid rather than anything related to the lungs or immune system.
A study specifically tracking patients with Graves’ disease during treatment found that those who entered a hypothyroid state had ACE levels significantly below the normal control group.6PubMed Central. Changes of Serum Angiotensin-Converting Enzyme Activity During Treatment of Patients with Graves’ Disease This means that even people being treated for hyperthyroidism can temporarily develop low ACE if treatment overshoots and pushes them into the hypothyroid range.
Corticosteroids Suppress ACE Levels
Corticosteroid drugs like prednisone reliably push ACE levels down. This has been studied most thoroughly in sarcoidosis patients, where steroids are a common treatment. Research found a strong inverse relationship between steroid dose and ACE level: when steroid doses went up, ACE fell, and when doses were tapered, ACE climbed.7PubMed. Effects of sarcoid and steroids on angiotensin-converting enzyme A separate study noted that in patients whose ACE was already at the low end of normal, prednisone could not push it down any further, while those starting with elevated levels saw a clear decline over one to four weeks of treatment.8PubMed. The influence of prednisone on serum angiotensin-converting enzyme activity in patients with and without sarcoidosis For anyone taking oral steroids for asthma, autoimmune conditions, or inflammatory bowel disease, this is worth knowing: the steroid itself can produce a low ACE result.
Zinc Deficiency and Fasting
ACE is a zinc-dependent enzyme. It literally needs zinc atoms at its active site to function. When dietary zinc drops sharply, ACE activity falls with it. Animal studies showed that zinc-deprived rats and guinea pigs had ACE activities in serum of roughly 42% and 59% of control values, respectively. Adding zinc back to the assay restored enzyme function, and removing it again with a chelating agent knocked the enzyme out, an effect completely reversible by re-adding zinc.9PubMed. Effects of dietary zinc deprivation on the activity of angiotensin-converting enzyme in serum of rats and guinea pigs Severe zinc deficiency in humans is uncommon in well-nourished populations but does occur in people with malabsorption syndromes, chronic alcoholism, very restrictive diets, and certain gastrointestinal diseases. In these individuals, low ACE levels could reflect nutritional status rather than any specific organ pathology.
Fasting itself also seems to lower ACE. A study measuring ACE activity in people who fasted found a progressive drop over the fasting period, and the researchers concluded that this reduction was not explained by changes in thyroid hormone (T3) levels.10PubMed. Angiotensin-converting enzyme activity decreases during fasting The practical implication is less dramatic than it sounds, because routine fasting before a blood draw is short, but prolonged caloric restriction could meaningfully affect ACE results.
Preeclampsia and Pregnancy-Related Changes
Pregnancy reshapes the renin-angiotensin system extensively. In a normal pregnancy, plasma ACE levels actually rise significantly compared to non-pregnant women. But in women who develop preeclampsia, ACE levels are significantly lower than in women with uncomplicated pregnancies.11PubMed Central. Angiotensin Converting Enzyme 2 (ACE2) in Pregnancy: Preeclampsia and Small for Gestational Age ACE2 levels and activity follow the same pattern, dropping in preeclampsia compared to normal pregnancy. The drop in ACE in preeclampsia is part of a broader dysregulation of blood-pressure control mechanisms that contributes to the dangerously high blood pressure, kidney stress, and other complications of the condition. If you’re pregnant and see an ACE level on your lab work, the result needs to be interpreted within the context of pregnancy physiology, not compared to standard reference ranges for non-pregnant adults.
Autoimmune and Inflammatory Conditions
While granulomatous diseases like sarcoidosis typically raise ACE (because activated immune cells called granulomas produce extra enzyme), some autoimmune conditions push ACE in the opposite direction. Research on patients with systemic lupus erythematosus (SLE) and systemic sclerosis found that ACE activity in their blood was lower than in healthy controls. Differences were also observed between subgroups of systemic sclerosis patients depending on whether their disease was diffuse or limited. The mechanism is not entirely clear, but may involve autoantibodies that target components of the renin-angiotensin system.
This creates an interesting diagnostic wrinkle. A doctor ordering an ACE level to screen for sarcoidosis in a patient who also has lupus might see a “normal” or low result and conclude sarcoidosis is unlikely, when in reality the autoimmune condition could be masking what would otherwise be an elevated level. This is one of many reasons the ACE test alone is a poor screening tool.
The Sarcoidosis Paradox and Why Low ACE Matters Diagnostically
Most people encounter the ACE test because a doctor suspects sarcoidosis or is monitoring an existing sarcoidosis diagnosis. The conventional teaching is that sarcoidosis raises ACE levels because granulomas overproduce the enzyme. That part is true, but the test is far less reliable than many patients (and some clinicians) assume. A large retrospective study found that elevated ACE levels had a sensitivity of only about 64% for sarcoidosis, meaning more than a third of confirmed sarcoidosis patients had normal or low ACE. The positive predictive value was roughly 13%, so the vast majority of people with a high ACE level did not actually have sarcoidosis. On the other hand, the negative predictive value was about 95%, meaning a normal or low ACE made sarcoidosis unlikely, though still possible.12PubMed Central. Put Down the ACE: Low Clinical Utility for Angiotensin-Converting Enzyme Levels in Sarcoidosis: A Single-Center Retrospective Cohort Study
In practice, this means a low ACE result is moderately reassuring if sarcoidosis is the concern, but it doesn’t rule it out entirely. Among confirmed sarcoidosis patients, disease chronicity appears to matter. A study using high-resolution CT scans found that chronic sarcoidosis patients were more likely to have atypical imaging findings, and the relationship between ACE levels and disease stage was not straightforward.13PubMed Central. Serum ACE Level in Sarcoidosis Patients with Typical and Atypical HRCT Manifestation Some patients with genuine, active sarcoidosis simply do not produce elevated ACE, especially those on corticosteroid treatment, those with certain genetic variants, or those whose granulomatous burden is modest.
Liver Disease and ACE
The liver plays a role in ACE metabolism that is sometimes overlooked. Research on patients with various forms of liver disease found that ACE activity tended to increase in progressive order from chronic persistent hepatitis to chronic aggressive hepatitis to cirrhosis.14The American Journal of Medicine. Angiotensin-converting enzyme in diseases of the liver At first glance, this seems to be about high ACE, not low. But the relevance to low ACE is indirect and worth noting: in earlier or milder stages of liver disease, ACE levels may not rise and can remain in the normal-to-low range. And because the liver clears many substances from the blood, severe liver failure can sometimes produce unpredictable enzyme levels, including ones that are lower than expected for the degree of inflammation present. The relationship between liver disease and ACE is not as clean as the thyroid connection, but it adds another variable that clinicians should consider when interpreting a result.
Age and Developmental Factors
ACE levels are not static across a lifetime. Research examining the relationship between age and ACE activity found that age was a significant predictor, with older adults tending to have different levels than younger people.15Brain Research. Angiotensin converting enzyme serum activities: Relationship with Alzheimer’s disease Reference ranges for ACE testing were largely established in adult populations, and they may not apply well to children, adolescents, or very elderly individuals. Children and teenagers tend to have higher ACE levels than adults, which means a “normal” adult value could actually be relatively low for a young person. Labs do not always provide age-adjusted reference ranges, so a result that looks normal on paper might warrant closer attention depending on the patient’s age.
The Alzheimer’s Disease Connection
One of the more surprising associations with low ACE involves Alzheimer’s disease. A study comparing ACE levels in Alzheimer’s patients and healthy controls found that both protein levels and enzyme activity in cerebrospinal fluid and blood were lower in people with Alzheimer’s. Lower ACE in cerebrospinal fluid tracked with lower levels of amyloid-beta (indicating more amyloid buildup in the brain) and with lower tau and phosphorylated tau levels.16PubMed Central. The association of angiotensin-converting enzyme with biomarkers for Alzheimer’s disease
This does not mean low ACE causes Alzheimer’s, and nobody is screening for dementia with an ACE blood test. But it reflects a broader theme in ACE biology: the enzyme participates in many processes beyond blood-pressure regulation, including some that involve brain function and inflammation. The finding is at an early research stage, and what it means for patients remains unclear. Still, it’s a reminder that ACE is not just a blood-pressure molecule, and changes in its level can be markers of processes happening in unexpected organ systems.
When a Low ACE Result Actually Means Nothing
Given all the factors that can push ACE down, a low result on a routine lab panel often carries no clinical significance at all. Here’s a quick rundown of the situations where a low ACE level is expected and usually not worth worrying about:
- You take an ACE inhibitor: the drug directly suppresses the enzyme, making the test meaningless while you’re on the medication.
- You take corticosteroids: prednisone and similar drugs reliably lower ACE.
- You have hypothyroidism: low thyroid function drags ACE down, and it recovers when thyroid levels are corrected.
- Your genetics favor lower levels: some people naturally sit at the low end of the reference range because of their ACE gene variant.
- You fasted heavily before the draw: prolonged caloric restriction lowers ACE activity.
The most important practical step when you see a low ACE result is to work backward through these common explanations before assuming the result signals a disease process. Ask whether any of the medications, nutritional factors, or endocrine conditions listed above apply, because they frequently do.
Why the ACE Test Persists Despite Its Limitations
Given its poor sensitivity and specificity for the disease it’s most associated with, you might wonder why anyone still orders the ACE test. The honest answer is partly inertia and partly that it still has narrow uses. For patients with a confirmed sarcoidosis diagnosis, serial ACE levels over time (rather than a single snapshot) can give a rough sense of whether disease activity is increasing or decreasing, especially if the patient is not on steroids. A result that is high for that individual patient and then drops with treatment, or vice versa, carries more meaning than any single value compared to a population reference range.
But for initial diagnosis, the test has real problems. The study that reported a positive predictive value of roughly 13% for sarcoidosis underscores that an elevated result is far more likely to be a false alarm than a true positive.12PubMed Central. Put Down the ACE: Low Clinical Utility for Angiotensin-Converting Enzyme Levels in Sarcoidosis: A Single-Center Retrospective Cohort Study And a normal or low result still leaves open the possibility of sarcoidosis in about 5% of cases. The test is best treated as one small piece of a larger diagnostic puzzle, never as a standalone answer. Tissue biopsy and imaging remain the cornerstones of sarcoidosis diagnosis, with ACE levels playing a supporting role at most.
For anyone who has received a low ACE result and is unsure what to make of it, the most productive next step is reviewing your medication list, thyroid function, and nutritional status with your doctor, rather than assuming the result points to a specific disease. In most cases, the explanation is mundane.