Alkaline Phosphatase · low result
Low alkaline phosphatase (ALP): causes, what it means and what to check next
Short answer
Low alkaline phosphatase — below about 40 U/L in adults — is much less common than a high result and is usually mild and harmless, but it has a short list of real causes worth checking once. Hypothyroidism, zinc or magnesium deficiency (the enzyme needs both), poor nutrition, anemia from vitamin B12 deficiency, and bone-turnover drugs such as bisphosphonates can all lower it. Two rare conditions matter because they are easy to miss: hypophosphatasia, an inherited bone disorder in which ALP is persistently very low, and Wilson's disease, in which a very low ALP accompanies acute liver failure.
Medically reviewed by Youmna DiStefano, MD · Last reviewed October 2026
When does this result need same-day care?
- A very low ALP with jaundice, dark urine, pallor or confusion in a child, teenager or young adult — Wilson's disease presenting as liver failure is a same-day emergency
- A new fracture from a minor injury or a foot that will not stop hurting with a known low ALP — stress fractures in hypophosphatasia heal poorly if missed
What is the normal range for Alkaline Phosphatase?
- age 18+: 40.0 – 129 U/L
Can reflect malnutrition, low zinc, or rare genetic conditions.
Full reference table, citations and testing options: Alkaline Phosphatase biomarker page.
What are the common causes of low Alkaline Phosphatase?
- Hypothyroidism — bone turnover slows and ALP falls; it recovers with thyroid replacement
- Zinc or magnesium deficiency — both are cofactors the enzyme needs to work; poor diet, alcohol, gut disease, bariatric surgery and some diuretics are the usual routes
- Malnutrition and protein deficiency, including anorexia and malabsorption from celiac disease or inflammatory bowel disease
- Vitamin B12 or folate deficiency (pernicious anemia, megaloblastic anemia), vitamin C deficiency (scurvy) and, less often, vitamin B6 excess
- Medications that suppress bone turnover — bisphosphonates (alendronate, zoledronic acid), denosumab — and, to a lesser degree, estrogen or combined oral contraceptives and glucocorticoids
- Hypophosphatasia — an inherited deficiency of the enzyme itself (ALPL gene); in adults it shows as recurrent stress fractures in the feet, early loss of adult teeth, poorly healing fractures, joint pain from calcium crystal deposits and a persistently low ALP since childhood
- Wilson's disease presenting as acute liver failure — a very low ALP together with hemolytic anemia and a high bilirubin is a classic, urgent pattern in a young person
- Transient falls after cardiac bypass surgery or large blood transfusions (citrate binds the zinc and magnesium the enzyme needs), hypoparathyroidism, achondroplasia, and simply the lower end of normal variation in a small, slight adult
What symptoms go with low Alkaline Phosphatase?
- None in most cases — a low ALP is an incidental finding
- Symptoms of the cause: fatigue, cold intolerance and weight gain with hypothyroidism; poor appetite, hair loss, slow wound healing or altered taste with zinc deficiency; numbness or memory problems with B12 deficiency
- With hypophosphatasia: foot pain from metatarsal stress fractures, adult teeth that loosen and fall out early, bone and joint pain, fractures that take unusually long to heal
- With Wilson's disease: jaundice, dark urine, fatigue and confusion in a child, teenager or young adult
What do clinicians usually check next?
- Repeat it — a single low value in someone well is often normal variation; two low values deserve the short workup below
- TSH and free T4 to exclude hypothyroidism, the most common reversible cause
- Zinc and magnesium levels, vitamin B12 and folate, and a CBC for anemia
- Calcium, phosphorus, 25-hydroxyvitamin D and PTH — in hypophosphatasia calcium and phosphorus run high-normal or high while vitamin D is normal, the opposite of the pattern in vitamin D deficiency
- A medication review for bisphosphonates, denosumab, estrogen and steroids
- If ALP has been low repeatedly and there is a history of stress fractures, early tooth loss or a relative with brittle bones: a serum pyridoxal 5'-phosphate (vitamin B6) level, which is high in hypophosphatasia, and genetic testing of the ALPL gene — an enzyme-replacement treatment exists, so the diagnosis matters
- In a young person with a low ALP and abnormal liver tests or hemolysis: ceruloplasmin, serum and 24-hour urine copper, and a slit-lamp eye exam for Wilson's disease
Follow-up tests available at TestWell: CBC with Differential Test ($17.99), TSH Test (Thyroid Stimulating Hormone) ($20.99), TSH with Reflex to Free T4 Test ($29.99), TSH + Free T4 Test ($27.99), Vitamin B12 Test ($19.99), Magnesium Test ($18.99).
How do you recheck Alkaline Phosphatase?
Prices are cash-pay; every order adds one $6 lab processing fee. Physician authorization is included; no insurance or referral needed.
| Test or panel | Type | Price | Fasting | Results |
|---|---|---|---|---|
| Liver Health Panel | Panel · 4 tests | $59.99 | No | 2–5 business days |
| Fatty Liver Panel | Panel · 6 tests | $92.99 | Yes | 2–4 business days |
| Post-COVID Recovery Panel | Panel · 7 tests | $99.99 | Yes | 2–4 business days |
| Liver Function Panel | Test · 7 biomarkers | $19.49 | No | 1–3 business days |
Common questions
What does a low alkaline phosphatase level mean?
That there is less of the enzyme in the blood than usual — below about 40 U/L in most adult ranges, though some labs set the floor at 30 or 44. In most people it means little: the enzyme level varies, and a slight adult with low bone turnover can sit under the line and be entirely healthy. When it is persistently low, the usual explanations are hypothyroidism, a shortage of zinc or magnesium, poor nutrition, B12 deficiency or a bone drug such as a bisphosphonate. Rarely it is the clue to an inherited enzyme deficiency called hypophosphatasia.
Is low alkaline phosphatase serious?
Usually not, and a mildly low value with no symptoms needs only a repeat and a few simple blood tests. It becomes significant in two settings. One is hypophosphatasia — ALP that has been low for years in someone with stress fractures, early tooth loss or bone pain — because a treatment exists and the fractures heal badly without the diagnosis. The other is a very low ALP in a young person with acute liver problems and anemia, which can signal Wilson's disease and is an emergency.
Which vitamin or mineral deficiency causes low ALP?
Zinc and magnesium most directly, because the enzyme cannot function without them; low intake, alcohol, gut disease, bariatric surgery and some medications deplete them. Vitamin B12 and folate deficiency lower ALP by reducing bone-cell activity, vitamin C deficiency (scurvy) does the same, and protein malnutrition lowers it generally. Checking zinc, magnesium, B12 and folate together, and correcting whatever is low, is the standard approach; the ALP recovers over a few months.
Can medications cause a low ALP?
Yes. Bisphosphonates such as alendronate and zoledronic acid, and denosumab, are designed to slow bone turnover, and a lower ALP is an expected sign that they are working — many people on them for osteoporosis sit below the adult range. Estrogen and combined oral contraceptives, glucocorticoids, clofibrate and some chemotherapy agents lower it to a lesser degree. A low ALP in someone on these drugs needs no action unless there is another reason to suspect a problem.
What is hypophosphatasia?
An inherited condition in which the gene for the tissue-nonspecific alkaline phosphatase (ALPL) does not work normally, so bones and teeth mineralize poorly. Severe forms appear in infancy; the mild adult form is often missed for decades and shows up as recurrent metatarsal stress fractures, early loss of adult teeth with the root intact, poorly healing fractures, calcium crystal arthritis and chronic muscle and bone pain — with an ALP that has always been low. A high serum vitamin B6 (pyridoxal 5'-phosphate) level supports the diagnosis and genetic testing confirms it. An enzyme-replacement therapy (asfotase alfa) is approved for the childhood-onset forms.
My ALP is 35 — is that a problem?
On its own, almost certainly not. A value a few points under the lab's floor in someone with normal thyroid, calcium and blood counts and no history of fractures or dental problems is normal variation for many people, particularly smaller adults and anyone on a bisphosphonate. A repeat in a few months, and a TSH plus zinc, magnesium and B12 if it stays low, is as far as it needs to go unless the fracture or dental history raises the hypophosphatasia question.
Educational content, not a diagnosis. Lab results are one input; a clinician interprets them with your history and exam.