Parathyroid Hormone (PTH) Unit Converter
Parathyroid Hormone (PTH) Unit Converter
Convert intact PTH between pg/mL, pmol/L and ng/L, and see why a PTH from one laboratory cannot be compared with a PTH from another.
Parathyroid Hormone (PTH) converter
Mass ⇄ molarIntact PTH 35 pg/mL
Formula and conversion factors
pg/mL = pmol/L ÷ 0.106101
ng/L = pg/mL (numerically identical)
- 0.106101
- derived from the molecular mass of intact PTH(1-84), 9,425 Da
- ng/L
- numerically identical to pg/mL; one nanogram per litre is one picogram per millilitre, so no conversion is needed
- PTH(1-84)
- the factor applies to the intact 84-amino-acid hormone; circulating fragments have different masses and are detected differently by different assays
- generation
- second-generation intact assays also detect the large PTH(7-84) fragment, third-generation whole-PTH assays largely do not, so the two read differently on the same sample
Worked example
Intact PTH 35 pg/mL
35 × 0.106101 = 3.7 pmol/L
= 35.0 ng/L — pg/mL and ng/L are the same number
Within the adult interval of 1.6–6.9 pmol/L, which is 15–65 pg/mL
Intact PTH across the three units
| pg/mL | pmol/L | ng/L |
|---|---|---|
| 10 | 1.1 | 10.0 |
| 15 — typical lower limit | 1.6 | 15.0 |
| 35 | 3.7 | 35.0 |
| 65 — typical upper limit | 6.9 | 65.0 |
| 150 | 15.9 | 150.0 |
| 300 | 31.8 | 300.0 |
PTH read against the calcium
| Calcium | PTH | Interpretation | Next step |
|---|---|---|---|
| Raised | Raised or inappropriately normal | Primary hyperparathyroidism | 24-hour urine calcium and a calcium:creatinine clearance ratio to exclude familial hypocalciuric hypercalcaemia |
| Raised | Suppressed | PTH-independent hypercalcaemia | Look for malignancy, granulomatous disease, vitamin D toxicity or thiazides |
| Low or normal | Raised | Secondary hyperparathyroidism | Check 25-hydroxyvitamin D, renal function and dietary calcium |
| Low | Low or inappropriately normal | Hypoparathyroidism | Check magnesium — depletion causes a functional and reversible hypoparathyroidism |
One patient, one assay — why PTH does not travel between laboratories
Intact PTH is reported in pg/mL in the United States and in pmol/L elsewhere, with a factor of 0.106101 between them derived from the molecular mass of the intact 84-amino-acid hormone, 9,425 daltons. A PTH of 35 pg/mL is 3.7 pmol/L. The third unit that appears on reports, ng/L, is numerically identical to pg/mL and needs no conversion at all, because one nanogram per litre is one picogram per millilitre.
The arithmetic is the easy part. PTH circulates not only as the intact hormone but as a family of fragments, and immunoassays differ in which of them they detect. Second-generation intact PTH assays, which are what most laboratories run, also pick up the large carboxy-terminal fragment PTH(7-84); third-generation whole-PTH or bio-intact assays are designed to exclude it. The two generations therefore return different numbers on the same sample, and the gap widens in renal failure, where fragments accumulate.
Even within one generation, results are not harmonised. Different manufacturers’ intact PTH assays can differ by a factor of two or more on identical material, which is why external quality assessment schemes report results by method rather than pooling them and why reference intervals are method-specific. The practical consequence is simple and important: a patient’s PTH should be trended on one assay in one laboratory. A rise or fall seen across a change of provider may be entirely analytical.
Interpretation always requires a simultaneous calcium. A raised calcium with a PTH that is high or even merely within the reference interval is inappropriate and indicates primary hyperparathyroidism; a raised calcium with a properly suppressed PTH points to malignancy or another PTH-independent cause. A raised PTH with a normal or low calcium is secondary, most often from vitamin D deficiency or chronic kidney disease. In CKD the reference interval is not the target at all: KDIGO advises acting on the trend, with values aimed well above the normal range in dialysis patients.
Frequently asked questions
How do I convert PTH from pg/mL to pmol/L?
Multiply by 0.106101, a factor derived from the molecular mass of intact PTH(1-84), 9,425 Da. A PTH of 35 pg/mL is 3.7 pmol/L. To go the other way, divide the pmol/L figure by 0.106101.
Is pg/mL the same as ng/L for PTH?
Yes, exactly. One nanogram per litre is one picogram per millilitre, so the two are numerically identical and no conversion is needed between them.
Can I compare PTH results from two different laboratories?
No, not reliably. Second-generation intact assays also detect the PTH(7-84) fragment while third-generation whole-PTH assays largely do not, and even within one generation different manufacturers can differ by a factor of two on the same sample. A patient’s PTH should be trended on a single assay in a single laboratory.
What does a normal PTH with a high calcium mean?
It is abnormal. A high calcium should suppress PTH almost completely, so a PTH sitting inside the reference interval is inappropriately unsuppressed and is the usual biochemical picture of primary hyperparathyroidism. The next step is a 24-hour urine calcium with a calcium:creatinine clearance ratio to exclude familial hypocalciuric hypercalcaemia.
Does the normal range apply in chronic kidney disease?
No. In CKD, and particularly on dialysis, PTH is expected to run above the reference interval, and KDIGO guidance is to act on the trend of serial values rather than on a single result against the normal range. Both an excessively suppressed and a markedly rising PTH warrant a change of management.
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References
- Kidney Disease: Improving Global Outcomes (KDIGO) CKD-MBD Update Work Group. 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.
- Bilezikian JP, Khan AA, Silverberg SJ, 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.
- Sturgeon CM, Sprague SM, Metcalfe W. Variation in parathyroid hormone immunoassay results — a critical governance issue in the management of chronic kidney disease. Nephrol Dial Transplant. 2011;26(11):3440–3445.