What Does This Test Measure?
Parathyroid hormone is made by four small glands sitting behind the thyroid. Its single job is to keep blood calcium within a very narrow range, and it does that through three levers: pulling calcium out of bone, increasing calcium reabsorption in the kidney, and switching on the enzyme that converts vitamin D into its active form, which in turn increases calcium absorption from the gut.
The test almost always measures intact PTH, meaning the whole, biologically active hormone rather than the inactive fragments that also circulate. This matters in kidney disease, where fragments accumulate and older assays could overstate the true hormone level.
PTH is a feedback hormone, and that is the key to reading it. A PTH value is close to meaningless on its own. What carries the information is whether PTH is behaving appropriately for the calcium level measured at the same time. A PTH of 65 is perfectly normal alongside a low-normal calcium, and distinctly abnormal alongside a calcium of 11.
Roughly 15–65 pg/mL for intact PTH, though assays differ enough that your laboratory's own interval should be used. There is no meaningful sex difference. PTH drifts upward with age, largely because vitamin D status and kidney function decline. In chronic kidney disease, targets are deliberately set above the normal range and vary by disease stage, so a raised PTH in that setting is not read the same way.
Why Your Doctor Ordered This Test
The overwhelmingly common reason is an abnormal calcium found on a routine panel. Once calcium is high or low, PTH is the single test that splits the diagnosis in half, which is why it is rarely ordered alone and rarely useful alone.
It is also checked when someone has osteoporosis or unexplained bone loss, recurrent kidney stones, chronic kidney disease, symptoms suggesting hypercalcemia such as fatigue, constipation, excessive thirst and confusion, or after thyroid or parathyroid surgery where the glands may have been injured.
What High PTH Means
High PTH with high calcium is primary hyperparathyroidism until proven otherwise. One of the four glands has become autonomous, usually a single benign adenoma, and is producing hormone without regard to the feedback signal. It is far more common in women after 50, and is now most often found incidentally on a routine panel in someone with no symptoms at all. Familial hypocalciuric hypercalcemia mimics it closely and is excluded with a urine calcium measurement.
High PTH with low or normal calcium is secondary hyperparathyroidism, and here the glands are behaving correctly. Something else is dragging calcium down and PTH is compensating. Vitamin D deficiency is by far the commonest cause, followed by chronic kidney disease, malabsorption from celiac disease or bariatric surgery, and low dietary calcium. Treating the underlying cause brings PTH back down.
Long-standing secondary hyperparathyroidism can progress to tertiary disease, where the chronically stimulated glands stop responding to feedback entirely and calcium rises. This is mainly seen in advanced kidney disease and after transplantation.
What Low PTH Means
Low PTH with high calcium means the glands are correctly switching off, and the calcium is coming from somewhere else. The two causes that matter are malignancy (either bone metastases or a tumour secreting PTH-related peptide, which does not register on a PTH assay) and granulomatous disease such as sarcoidosis, where activated macrophages convert vitamin D independently of PTH. Excess vitamin D or calcium supplementation produces the same pattern more benignly.
Low PTH with low calcium is hypoparathyroidism, meaning the glands themselves have failed. The commonest cause by a wide margin is damage or removal during thyroid or neck surgery. Autoimmune destruction, severe magnesium depletion, and rare genetic syndromes account for most of the rest. Magnesium deserves particular attention: severe deficiency physically blocks PTH release, and the resulting low calcium will not correct with calcium replacement alone until magnesium is restored.
How to Prepare and What Affects the Result
Fasting is not usually required, but PTH has a daily rhythm that peaks overnight and dips in the late morning, so a morning draw makes serial results comparable. The single most important requirement is that calcium is measured on the same sample, because the pair is what gets interpreted. Albumin should be measured alongside so that calcium can be corrected.
Vitamin D supplements, calcium supplements, lithium and thiazide diuretics all shift results and should be noted. Biotin supplements can interfere with some immunoassays and are worth stopping for a couple of days beforehand. PTH is unstable at room temperature, so a delayed or badly handled sample can read falsely low.
Common Misinterpretations
The most frequent error is reading PTH on its own. A result inside the reference range is often described as normal when, alongside a calcium of 10.9, it is entirely inappropriate; a healthy gland should have shut down. This inappropriately normal PTH is the classic presentation of mild primary hyperparathyroidism and is missed constantly.
The second is assuming a high PTH means parathyroid disease. Far more often it is a normal gland responding correctly to vitamin D deficiency, and the treatment is vitamin D, not surgery. Checking vitamin D before acting on a raised PTH prevents a great deal of unnecessary investigation.
The third is applying standard reference ranges in chronic kidney disease, where a PTH well above the normal range is the expected and accepted target rather than a finding to correct.
Frequently Asked Questions
Can vitamin D deficiency cause high PTH?
Yes, and it is the commonest cause of a raised PTH. When vitamin D is low, less calcium is absorbed from the gut, calcium drifts down, and the parathyroid glands increase output to compensate. This is a normal gland doing its job. Correcting vitamin D usually brings PTH back into range over several months, which is why vitamin D should always be checked before a raised PTH is investigated further.
What does a normal PTH with high calcium mean?
It is often abnormal despite looking normal. Healthy parathyroid glands should switch off almost completely when calcium is high, so a PTH sitting in the middle of the range alongside a raised calcium is inappropriate. This pattern is the typical presentation of mild primary hyperparathyroidism and warrants further assessment rather than reassurance.
Do I need surgery if I have primary hyperparathyroidism?
Not always. Surgery is clearly indicated with significantly raised calcium, kidney stones, reduced kidney function, osteoporosis, or in younger patients. Many older people with mild, symptom-free disease are monitored instead, with periodic calcium, kidney function and bone density checks. It is a decision made with an endocrinologist based on the whole picture, not on the PTH number alone.
Why does my doctor keep checking magnesium with my PTH?
Because severe magnesium deficiency physically prevents the parathyroid glands from releasing PTH. The result is low calcium with a low or inappropriately normal PTH that will not respond to calcium supplementation. Once magnesium is replaced, PTH release resumes and calcium corrects, often quite quickly.
Does time of day affect a PTH result?
Somewhat. PTH follows a daily rhythm with an overnight peak and a late-morning trough, and the swing can be meaningful when values sit near a decision threshold. It rarely changes a clear-cut result, but taking repeat samples at a consistent morning time makes trends easier to interpret.
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