- 01PTH is half of a two-number test
A PTH value has no fixed meaning. It becomes a finding only when you read it against a calcium drawn from the same blood; the four calcium-and-PTH combinations are what name the problem, not the PTH number by itself.
- 02Same PTH, opposite meaning
A high PTH next to a high calcium points to a parathyroid that has gone rogue. The identical high PTH next to a low or normal calcium points to a perfectly healthy parathyroid doing its job, compensating for low vitamin D or failing kidneys.
- 03A 'normal' PTH can still be wrong
This is the same two-number rule from the other side: when calcium is high, the parathyroid should switch off and PTH should fall toward the floor. A PTH sitting in the normal range while calcium is high is inappropriately normal, and it still points back to the parathyroid.
Parathyroid hormone (PTH) is your body's calcium thermostat. When blood calcium drops, four small glands in your neck, behind the thyroid, release PTH to bring it back up. That is why a PTH result on its own tells you almost nothing. Its meaning flips depending on the calcium sitting beside it, so PTH and calcium are really one test read together.
What is parathyroid hormone (PTH)?
PTH is a hormone made by the four parathyroid glands, each about the size of a grain of rice, tucked behind your thyroid in the front of your neck. Their one job is to defend the level of calcium in your blood, minute to minute. When you get a PTH blood test, the lab is measuring intact PTH, the active full-length hormone, usually reported in pg/mL (picograms per milliliter).
What PTH does in the body
Calcium does more than build bone; it runs your nerves, muscles, and heartbeat, so your body keeps it in a tight range and reacts fast when it slips. PTH is the lever. When calcium falls, PTH rises and raises it three ways at once: it frees calcium from bone, tells the kidney to hold on to calcium and flush out phosphate, and switches on vitamin D so your gut absorbs more calcium from food. 1 When calcium climbs back to normal, the glands sense it and switch PTH off.
How PTH keeps blood calcium steady
What is a normal PTH level?
Intact PTH usually runs about 15 to 65 pg/mL, though every lab sets its own range, so read the range printed on your own report. 1 But normal is not a verdict on its own. Whether a given PTH is right or wrong depends entirely on the calcium measured at the same time.
| Intact PTH | Label | What it suggests (always read with calcium) |
|---|---|---|
| Below ~15 pg/mL | Low / suppressed | The parathyroid is turned down. Appropriate if calcium is high; a problem if calcium is low (hypoparathyroidism, too little PTH). |
| ~15 to 65 pg/mL | Normal reference range | In range, yet a mid-range PTH can be inappropriately normal: alongside a high calcium it still points to an overactive parathyroid. |
| Above ~65 pg/mL | High / elevated | The parathyroid is working hard. A primary gland problem if calcium is high; compensation for low vitamin D or kidney disease if calcium is low or normal. |
Read PTH and calcium as one test
To read PTH, find the calcium drawn from the same blood sample and read the two together. PTH exists to keep calcium steady, so what matters is whether PTH is doing the right thing for this calcium. The four combinations below are what name the problem.
| Calcium | PTH | Most likely picture | Common next step |
|---|---|---|---|
| High | High or high-normal | Primary hyperparathyroidism: one gland overproducing on its own, usually a single benign growth | Clinician repeats calcium and PTH, checks vitamin D and 24-hour urine calcium |
| High | Low (suppressed) | Hypercalcemia (high blood calcium) from outside the parathyroid, often cancer-related; the gland has correctly shut off | Prompt clinician workup for the source |
| Low or normal | High | Secondary hyperparathyroidism: a healthy gland compensating for low vitamin D or kidney disease | Check 25-OH vitamin D and kidney function |
| Low | Low | Hypoparathyroidism: too little PTH, often after neck or thyroid surgery, or autoimmune | Clinician evaluation; symptomatic low calcium can be urgent |
One caveat before you trust a calcium at all: the total calcium number can be pulled off by blood protein.
Total calcium tracks albumin, so a low total can be normal calcium hiding behind low protein; correct it before you worry.
So confirm the calcium is genuinely high or low (your clinician may correct it for albumin or order an ionized calcium) before you lean on the PTH pairing. The full detail lives on the total calcium page.
What does high PTH mean?
A high PTH means the glands are working hard, and the calcium tells you why. If calcium is also high, the gland itself is overproducing; that is primary hyperparathyroidism, usually caused by a single benign (noncancerous) growth on one gland called an adenoma. 1 If calcium is low or normal, the gland is compensating for something else, most often low vitamin D or reduced kidney function; that is secondary hyperparathyroidism. 15 A high PTH with a normal calcium is usually this compensating pattern, but it can occasionally be early, normocalcemic (normal-calcium) primary hyperparathyroidism, which is why a clinician confirms the picture with repeat labs, vitamin D, and kidney function. 1 The same high PTH points to opposite situations.
High PTH: primary or secondary hyperparathyroidism?
Low vitamin D is the most common reason for a high PTH with normal calcium. As 25-OH vitamin D drops below roughly 20 to 30 ng/mL, PTH climbs to keep calcium defended. 3 Correcting the deficiency often settles a mildly high PTH; ask your clinician about rechecking PTH a few months after starting vitamin D to confirm it has settled.
25-OH vitamin D is the storage form labs measure to gauge your status; it is the number that shows whether your reserves are running low.
What does low PTH mean?
A low or suppressed PTH also means different things depending on calcium. If calcium is high while PTH is low, the parathyroid is behaving correctly; it has switched off because something outside the gland is raising calcium. In an adult, that search often points toward a cancer-related cause, which is why a low PTH with high calcium is worked up promptly. 2 If calcium is low while PTH is also low, the glands are underproducing; that is hypoparathyroidism, most often after surgery on the thyroid or neck, or from an autoimmune cause.
Primary hyperparathyroidism and malignancy account for about 90% of all cases of hypercalcaemia.Minisola et al., BMJ, 2015 [[2]]
Two look-alikes worth knowing
Two things can mimic an overactive parathyroid. Familial hypocalciuric hypercalcemia (FHH), a harmless inherited condition, produces a mildly high calcium with a normal or slightly high PTH, exactly like mild primary hyperparathyroidism; a 24-hour urine calcium (low in FHH) tells them apart, which is why clinicians order it before considering surgery. 1 Certain medicines also nudge these numbers: thiazide diuretics (common blood-pressure pills) and lithium can raise calcium and PTH, so your clinician will weigh what you take.
How PTH is tested
PTH is a simple blood draw, and you do not usually need to fast for it. It is a separate order from a basic metabolic panel, so calcium and PTH sometimes come from different tubes; ask that they be drawn together, ideally the same morning, because the pair is the whole point. Labs often add 25-OH vitamin D and kidney-function tests to interpret a high PTH. The sample is handled with care because PTH degrades if it sits warm, so an occasional redraw is normal.
PTH is not a routine screening test. It is ordered to work up an abnormal calcium, low bone density, a vitamin D deficiency, or kidney disease, and it is part of the blood panel used to find the treatable causes behind bone loss rather than to diagnose osteoporosis itself. It sits within the wider bone health picture. 4 One flagged result is usually repeated to confirm it. If primary hyperparathyroidism turns up in someone young, it can run in families as part of an inherited syndrome, and relatives may be offered testing.
Link · NIDDK (National Institute of Diabetes and Digestive and Kidney Diseases)Primary hyperparathyroidismPlain-language patient reference on what happens when a parathyroid gland overproduces PTH, how it is found, and how it is managed.niddk.nih.gov
Citations
- Bilezikian JP, et al. Evaluation and Management of Primary Hyperparathyroidism: Summary Statement and Guidelines from the Fifth International Workshop. J Bone Miner Res. 2022;37(11):2293-2314.
- Minisola S, et al. The diagnosis and management of hypercalcaemia. BMJ. 2015;350:h2723.
- Holick MF, et al. Evaluation, Treatment, and Prevention of Vitamin D Deficiency: an Endocrine Society Clinical Practice Guideline. JCEM. 2011;96(7):1911-1930.
- LeBoff MS, et al. The clinician's guide to prevention and treatment of osteoporosis (BHOF). Osteoporos Int. 2022;33(10):2049-2102.
- KDIGO 2017 Clinical Practice Guideline Update for the Diagnosis, Evaluation, Prevention, and Treatment of Chronic Kidney Disease-Mineral and Bone Disorder (CKD-MBD). Kidney Int Suppl. 2017;7(1):1-59.
Educational context only, not medical advice or a diagnosis. Always discuss your results with a clinician.
Associations to explore with a clinician, not a diagnosis from a single number.