Low alkaline phosphatase: what a low ALP means and what to check

A low alkaline phosphatase (ALP) usually means a short-term dip after a recent infection, an operation, the contraceptive pill or hypothyroidism (an underactive thyroid), and it often comes back normal on a repeat test. ALP that stays low on repeat tests with no obvious cause can be hypophosphatasia, a lifelong inherited condition of bone and teeth, and that changes which osteoporosis drugs are safe ¹².

What ALP is, and why low is the quieter direction

Alkaline phosphatase (ALP) is an enzyme found mainly in the liver and in bone, with smaller amounts from the gut and, in pregnancy, the placenta. The standard blood test measures the total of all these forms together. Doctors order ALP mostly to look for problems that raise it, such as blocked bile ducts or fast bone turnover, so the high side has its own high ALP page and the marker itself is explained on the ALP marker page.

A low ALP gets far less attention: most reports mark it with a small L and nothing more. A 2023 review of the low-ALP workup in Calcified Tissue International pointed out that low values are often overlooked on routine panels, even when they persist ¹. Of the low results, only a persistent one with no cause needs further tests.

How low is low ALP? Ranges by age, and what 39 or 44 U/L means

Adult ALP reference ranges usually run from about 35 to 45 U/L at the bottom to roughly 105 to 130 U/L at the top, depending on the lab's method, your sex and your age ¹. That spread in the lower limit is why the same ALP of 40 U/L can be flagged low at one lab and pass without comment at another, and why 46 U/L or more is within range at almost every lab. Read your number against the range printed on your own report, and see how reference ranges are built for why they differ.

Children and teenagers run much higher ALP than adults because growing bone makes a lot of it, with peaks in infancy and at the pubertal growth spurt. A result that looks normal by adult standards can be low for a child, which is why the 2024 hypophosphatasia criteria use ranges matched to age and sex ³. In pregnancy, placental ALP pushes the total up, most of all in the third trimester. Some labs report ALP in µkat/L; multiply by 60 to get U/L.

The common reasons ALP comes back low on a blood test

A 2023 diagnostic review lists the usual secondary causes: hypothyroidism, malnutrition, low zinc or magnesium, glucocorticoids (steroid tablets such as prednisolone), estrogen therapy, antiresorptive bone drugs (osteoporosis medicines that slow bone breakdown, such as bisphosphonates and denosumab), recent cardiac surgery or a large transfusion, Wilson disease, and blood samples contaminated with EDTA, an anticlotting chemical from the wrong collection tube ¹. The table orders them roughly by how often each one turns up in everyday practice.

Hypothyroidism lowers bone turnover, so ALP falls with it ¹. If your TSH is high, the hypothyroidism page covers the condition; once a doctor diagnoses and treats it, often with levothyroxine, ALP tends to rise as thyroid levels are corrected. Gut conditions that block absorption, such as celiac disease, can lower ALP through low zinc and magnesium, and B12 deficiency appears on the same cause lists ¹.

Hypophosphatasia: the rare cause of low ALP worth ruling out

Hypophosphatasia (HPP) is the inborn error of metabolism that features low activity of the tissue-nonspecific isoenzyme of alkaline phosphatase (TNSALP).Michael P. Whyte, Nature Reviews Endocrinology, 2016 ²

In plain words: the body makes too little of the bone and liver form of ALP. Hypophosphatasia is caused by faulty copies of the ALPL gene, which makes that form. With too little working enzyme, a few substances it normally clears build up. One is the active form of vitamin B6 (PLP), which is why a B6 blood test helps. Another is pyrophosphate, which stops calcium settling into bone and teeth, so they end up softer and weaker ².

The severe infant forms are rare, estimated at about 1 in 300,000 births in Europe, but milder forms are far more common, around 1 in 6,370 by the same molecular estimate summarised in Whyte's review ². Adults with the mild form often go undiagnosed for decades. Clues include stress fractures or fractures that heal slowly, atypical femoral fractures, losing baby teeth early (before age 5, often with the root still attached), chronic muscle or joint pain, and calcium crystal deposits in joints ²³.

Diagnosis rests on ALP that stays low for age and sex plus supporting findings. The 2024 international criteria count a disease-causing ALPL variant, a raised vitamin B6 (PLP), low-trauma or atypical femoral fractures and early loss of baby teeth among them ³. PLP is the cheap first test. Vitamin B6 supplements raise PLP in anyone and can blur the result, so ask the doctor ordering the test whether to pause B6 (including in multivitamins and energy drinks) beforehand; a pause of about a week is common. Enzyme replacement with asfotase alfa is approved for pediatric-onset disease and is managed by specialists ².

Wilson disease and low ALP in liver failure

Wilson disease is a copper-storage disorder that can present as sudden liver failure in young adults. In that setting ALP is often strikingly low while bilirubin is very high. A Hepatology study of patients in acute liver failure found that an ALP to total bilirubin ratio below 4, combined with an AST to ALT ratio above 2.2, identified Wilson disease with 100% sensitivity and 100% specificity in that cohort ⁴. The ratio uses ALP in U/L and bilirubin in mg/dL; divide a bilirubin in µmol/L by 17.1 first. Outside acute illness, a low ALP alongside normal ALT, AST and GGT does not point to liver disease; the liver enzymes explainer shows how the liver markers fit together.

Does low alkaline phosphatase mean cancer?

Low ALP is not a recognised warning sign of cancer. Cancers that spread to bone or liver push ALP up, which is covered on the high ALP page. Chemotherapy, weight loss and serious illness can lower ALP for a time, but in those cases the cancer is already known and the low ALP reflects the treatment or illness ¹. If you have symptoms that worry you, raise them with your doctor whatever your ALP.

What to check next for low ALP, in order

1. 1A repeat ALP, usually a few weeks laterDoctors usually repeat a low ALP first, ideally at the same lab and not within a few weeks of surgery, a transfusion or a bad infection. Many low results are normal on the repeat, and a trend tells you more than one number; see why trends beat single readings and how to prepare for a blood test.
2. 2If ALP is still low, the common causes are checkedA doctor will typically review your medicines (estrogen, glucocorticoids, bone drugs) and may order TSH, plasma zinc, serum magnesium, vitamin B12, albumin, calcium, phosphate and vitamin D, plus a celiac antibody test (tTG-IgA) if you have gut symptoms ¹.
3. 3If ALP stays low with no cause, a vitamin B6 (PLP) testThis applies at any value below your lab's limit, not only very low ones. Labs often ask people to pause B6 supplements for about a week beforehand; ask your doctor before stopping anything you take. A high PLP with persistently low ALP points towards hypophosphatasia ³.
4. 4Confirmation with an ALPL gene test and a specialistAn endocrinologist or metabolic bone specialist can arrange genetic testing and decide whether any treatment is needed. The bone markers explainer covers the other bone tests they may add.

Can you raise ALP? Food, supplements and time

The fix for a low ALP is to find and treat its cause. Treated hypothyroidism, a recovered illness or a reviewed medicine usually lets ALP drift back into range over weeks to a few months. Where a blood test proves low zinc or magnesium deficiency, correcting it under a doctor's guidance can help, and food sources (meat, shellfish, legumes, nuts, seeds and whole grains) cover both minerals.

Two cautions apply. ALP does not track zinc status reliably ⁵, so a low ALP alone is not a reason to take zinc, and long-term high-dose zinc can cause copper deficiency. If hypophosphatasia is on the table, B6 supplements are usually paused before the PLP test, as your doctor or lab advises, because they can hide or mimic the result. More on bone health sits in the bone topic and the liver topic.

Supplements with evidence that bears on low ALP

Mixed evidenceZincHelps only when plasma zinc is proven low; ALP is not a reliable zinc gauge, and excess zinc lowers copper.In our shop

Early evidenceMagnesiumCan help correct a magnesium deficiency confirmed on a blood test, one of the listed secondary causes of low ALP; take it under a clinician's guidance, especially with kidney disease.In our shop

Low ALP in children and in pregnancy

Because children normally run high ALP, a value that is low for age is more significant than the same number in an adult. Signs that should prompt a check for hypophosphatasia include baby teeth falling out before age 5 with the root intact, bowed legs, poor growth, delayed walking or frequent fractures ²³. A child with a low-for-age ALP and any of these signs should be seen by their pediatrician, who can refer to a metabolic bone specialist. In pregnancy, placental ALP raises total ALP, especially in the third trimester, so a low ALP late in pregnancy is unusual and worth mentioning to the obstetrician or midwife.

If your ALP has been low on two tests with no cause found, ask your doctor whether a vitamin B6 (PLP) test is needed before starting any bisphosphonate or denosumab.

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