Calcitonin Unit Converter
Calcitonin Unit Converter
Convert calcitonin between pg/mL, ng/L and pmol/L — and read it as what it is: the tumour marker for medullary thyroid carcinoma, with a cut-off that is nearly twice as high in men as in women and a long list of benign reasons to be mildly raised.
Calcitonin converter
Mass ⇄ molarCalcitonin 5.0 pg/mL in a man
The conversion, and the two units that are one number
pg/mL = pmol/L ÷ 0.292517
because 0.292517 = 1 pg/mL (10⁻¹² g/mL) ÷ 3,418.6 g/mol, the mass of the 32-residue calcitonin peptide
- MW 3,418.6
- human calcitonin is a 32-amino-acid peptide with an N-terminal disulphide ring and an amidated C-terminus, cleaved from procalcitonin. That mass is what makes pg/mL and pmol/L differ by roughly a factor of 3.4
- pg/mL = ng/L
- numerically identical. A calcitonin of 5 pg/mL is 5 ng/L, and most reports use pg/mL or ng/L rather than the molar unit
- pmol/L ≈ pg/mL ÷ 3.4
- a serviceable mental conversion: 14.3 pg/mL, the upper limit for men on Mayo's assay, is 4.18 pmol/L
- <b>not procalcitonin</b>
- procalcitonin is the 116-amino-acid precursor, measured in µg/L as a marker of bacterial sepsis, and it is a completely different test with a completely different purpose. The names are similar enough that requests and results are occasionally confused; the units are the giveaway
- assay-specific cut-offs
- commercial calcitonin immunoassays differ in their standardisation and their sensitivity at low concentrations, so the numerical cut-off belongs to the platform. Serial monitoring should stay on one method
Worked example
Calcitonin 5.0 pg/mL in a man
5.0 pg/mL = 5.0 ng/L — identical, no arithmetic required
5.0 × 0.292517 = 1.46 pmol/L
For a man, Mayo's upper limit is 14.3 pg/mL (4.18 pmol/L), so this is comfortably normal
Change one thing — the patient's sex — and the reading changes. For a woman the upper limit is 7.6 pg/mL (2.22 pmol/L), so a calcitonin of 9 pg/mL is normal in a man and raised in a woman. Men have roughly twice the C-cell mass
And before a raised value is acted on: is the patient on a proton pump inhibitor, are they smoking, what is the eGFR, and what is the calcium? Each of those lifts a basal calcitonin without any tumour being present
Reference values by age and sex, Mayo Clinic Laboratories, Roche ECLIA
| Group | pg/mL | pmol/L |
|---|---|---|
| Male, 17 years and over | ≤14.3 | ≤4.18 |
| Female, 17 years and over | ≤7.6 | ≤2.22 |
| 16 years | ≤5.0 | ≤1.46 |
| 10–15 years | ≤6.0 | ≤1.76 |
| 3–9 years | ≤7.0 | ≤2.05 |
| 2 years | ≤8.0 | ≤2.34 |
| 12–14 months | ≤14.0 | ≤4.10 |
| 6 months | ≤22 | ≤6.44 |
| 1 month | ≤34 | ≤9.95 |
Why a modestly raised calcitonin is usually not medullary thyroid carcinoma
| Cause | Mechanism | Typical magnitude |
|---|---|---|
| Proton pump inhibitors | Hypergastrinaemia stimulates C cells | Mild elevation, common; often resolves within weeks of stopping the drug |
| Chronic kidney disease | Reduced clearance of the peptide, and secondary hyperparathyroidism | Mild to moderate, rising as eGFR falls |
| Smoking | C-cell stimulation; a consistent, dose-related association | Mild |
| C-cell hyperplasia | Increased C-cell mass without malignancy — reactive, or a precursor lesion in hereditary disease | Mild to moderate |
| Hypercalcaemia, hyperparathyroidism | Calcium is the physiological stimulus to calcitonin release | Mild to moderate |
| Autoimmune (Hashimoto) thyroiditis | C-cell involvement in the inflamed gland | Mild |
| Other neuroendocrine tumours — lung, pancreatic, phaeochromocytoma | Ectopic production | Variable, occasionally marked |
| Heterophile antibody interference | Assay artefact | Any magnitude, including strikingly high |
| Medullary thyroid carcinoma | Tumour secretion, roughly proportional to tumour burden | A basal calcitonin above about 100 pg/mL is strongly suggestive; values in the thousands accompany established disease |
Basal, stimulated, and what calcitonin is used for
| Setting | How calcitonin is used | Alongside |
|---|---|---|
| Equivocal basal result | Stimulated test — calcium infusion (2.5 mg/kg elemental calcium) or, where still available, pentagastrin, with calcitonin sampled at intervals afterwards. An exaggerated peak supports C-cell disease; pentagastrin has been withdrawn in most countries, so calcium stimulation is the usual method | Repeat basal calcitonin on the same assay, CEA, calcium, renal function, drug history |
| Pre-operative assessment of a known MTC | Basal concentration correlates with tumour size and predicts nodal involvement; very high values raise the likelihood of distant disease | CEA, calcium, plasma metanephrines to exclude a phaeochromocytoma before surgery in MEN2, RET testing |
| Post-operative follow-up | An undetectable basal calcitonin indicates biochemical cure. A detectable or rising value indicates persistent or recurrent disease before imaging shows anything | CEA — the co-marker; a rising CEA with a plateauing or falling calcitonin suggests dedifferentiation and a worse prognosis |
| Calculating doubling time | Calcitonin and CEA doubling times of under six months are among the strongest predictors of progression and survival in medullary thyroid carcinoma | Serial measurements on one assay, at least four points over two years |
| Hereditary MTC and MEN2 families | Used for biochemical monitoring, but RET genotype — not calcitonin — determines who has prophylactic thyroidectomy and when | RET mutation testing, calcium, metanephrines |
A C-cell tumour marker that happens to live in the thyroid
Calcitonin is reported in picograms per millilitre, in nanograms per litre and occasionally in picomoles per litre. The first two are numerically identical. The molar conversion runs through the mass of the 32-amino-acid peptide, 3,418.6 daltons: one picogram per millilitre is 0.292517 picomoles per litre, so 5 pg/mL is 1.46 pmol/L and dividing a pg/mL figure by roughly 3.4 gets you there. A separate warning about names: calcitonin is not procalcitonin. Procalcitonin is the 116-residue precursor, measured in micrograms per litre as a marker of bacterial infection, and the two tests share nothing but a stem.
Physiologically calcitonin lowers serum calcium by inhibiting osteoclastic bone resorption, but in humans it is close to redundant: total thyroidectomy removes every C cell and causes no disorder of calcium, and a medullary carcinoma can produce enormous quantities without causing hypocalcaemia. Clinically, therefore, calcitonin is not a test of calcium metabolism and it is emphatically not a thyroid function test. It is a tumour marker. It is produced by the parafollicular C cells, which are the cells medullary thyroid carcinoma arises from, and its concentration is roughly proportional to tumour burden — which makes it unusually good at what it does, detecting persistent disease after surgery long before any imaging can.
Two things have to be right before a number is acted on. The first is that the cut-off must be sex-specific. Men have roughly twice the C-cell mass of women, and the reference limits reflect it: on Mayo’s electrochemiluminescence assay the adult upper limit is 14.3 pg/mL for men and 7.6 pg/mL for women. A calcitonin of 9 pg/mL is unremarkable in a man and raised in a woman, so applying one sex’s cut-off to the other either manufactures alarm or misses disease. Children sit higher again and fall through the first years of life, so paediatric results need paediatric limits.
The second is that a modestly raised calcitonin has a long and mostly benign differential. Proton pump inhibitors raise it through hypergastrinaemia; chronic kidney disease raises it through reduced clearance; smoking raises it; so do hypercalcaemia, hyperparathyroidism, autoimmune thyroiditis, reactive or hereditary C-cell hyperplasia, and neuroendocrine tumours arising elsewhere, while heterophile antibodies can produce a spuriously high result of any size. In the mildly raised range these causes together are far commoner than medullary carcinoma, which is the main argument against reflexively measuring calcitonin in every thyroid nodule and acting on the first abnormal value. The sequence that resolves an equivocal result is to confirm it on the same assay, review the drugs and the renal function, and then perform a stimulated test — calcium infusion, or pentagastrin where it is still available — since an exaggerated stimulated peak discriminates C-cell disease far better than a borderline basal value. A basal calcitonin above roughly 100 pg/mL is strongly suggestive on its own. In follow-up after thyroidectomy, calcitonin is always read with CEA, and the doubling times of the two markers are among the strongest available predictors of progression.
Frequently asked questions
How do you convert calcitonin from pg/mL to pmol/L?
Multiply by 0.292517, which is one picogram per millilitre divided by the mass of the 32-residue calcitonin peptide, 3,418.6 g/mol. So 5 pg/mL is 1.46 pmol/L, and dividing a pg/mL figure by roughly 3.4 is a good mental approximation. Picograms per millilitre and nanograms per litre are numerically identical, so no conversion is needed between those.
Why is the calcitonin reference range higher in men?
Because men have roughly twice the parafollicular C-cell mass of women, and calcitonin concentration tracks C-cell mass. On Mayo’s electrochemiluminescence assay the adult upper limit is 14.3 pg/mL for men and 7.6 pg/mL for women. The practical consequence is that a single unisex cut-off is wrong in both directions: a calcitonin of 9 pg/mL is normal in a man and raised in a woman. Sex-specific cut-offs are not a refinement, they are a requirement, and the same applies to children, whose values start high in infancy and fall through the first years of life.
Is calcitonin a thyroid function test?
No. It is made by the parafollicular C cells, which are embryologically distinct from the follicular cells that make thyroid hormone, and it says nothing at all about whether someone is hypothyroid or thyrotoxic. It is a tumour marker for medullary thyroid carcinoma, which arises from those C cells. Its physiological role — inhibiting osteoclastic bone resorption — is close to redundant in humans, which is why removing the whole thyroid causes no calcium disorder attributable to calcitonin deficiency.
My calcitonin is slightly raised. Does that mean cancer?
Usually not. In the mildly raised range the benign causes greatly outnumber medullary thyroid carcinoma: proton pump inhibitors, chronic kidney disease, smoking, hypercalcaemia and hyperparathyroidism, autoimmune thyroiditis, C-cell hyperplasia, neuroendocrine tumours elsewhere, and heterophile antibody interference with the assay. The first steps are to confirm that a sex-specific cut-off was used, to repeat the measurement on the same assay, and to review drugs and renal function. Where it remains equivocal, a stimulated test using calcium infusion — or pentagastrin where still available — discriminates far better than another basal measurement. A basal value above roughly 100 pg/mL is a different matter and is strongly suggestive.
What is a stimulated calcitonin test?
A provocation test used when a basal calcitonin is equivocal. Calcium is infused — typically 2.5 mg/kg of elemental calcium — or, historically, pentagastrin is given, and calcitonin is measured at intervals afterwards. C cells, and C-cell tumours in particular, release calcitonin briskly in response, so an exaggerated peak supports C-cell hyperplasia or medullary carcinoma while a flat response argues against it. Pentagastrin has been withdrawn in most countries, so calcium stimulation is now the usual approach; the thresholds are protocol- and assay-specific.
Why is CEA measured alongside calcitonin?
Because medullary thyroid carcinoma secretes both, and the two behave differently as the tumour evolves. After thyroidectomy, a rising calcitonin signals persistent or recurrent disease well before imaging does, and a rising CEA alongside it confirms it. The informative divergence is a rising CEA with a plateauing or falling calcitonin, which suggests dedifferentiation and carries a worse prognosis. Doubling times of under six months for either marker are among the strongest predictors of progression and survival, which is why both are trended on the same assay over years.
Related calculators
References
- Mayo Clinic Laboratories. Test ID: CATN — Calcitonin, Serum. Roche electrochemiluminescence immunoassay. Reference values, 17 years and older: males ≤14.3 pg/mL, females ≤7.6 pg/mL; paediatric values fall from ≤34 pg/mL at 1 month to ≤5.0 pg/mL at 16 years.
- Wells SA Jr, Asa SL, Dralle H, et al. Revised American Thyroid Association guidelines for the management of medullary thyroid carcinoma. Thyroid. 2015;25(6):567–610. doi:10.1089/thy.2014.0335
- Costante G, Meringolo D, Durante C, et al. Predictive value of serum calcitonin levels for preoperative diagnosis of medullary thyroid carcinoma in a cohort of 5817 consecutive patients with thyroid nodules. J Clin Endocrinol Metab. 2007;92(2):450–455. doi:10.1210/jc.2006-1590
- Toledo SPA, Lourenço DM Jr, Santos MA, et al. Hypercalcitoninemia is not pathognomonic of medullary thyroid carcinoma. Clinics (Sao Paulo). 2009;64(7):699–706. doi:10.1590/S1807-59322009000700015
- Barbot N, Calmettes C, Schuffenecker I, et al. Pentagastrin stimulation test and early diagnosis of medullary thyroid carcinoma using an immunoradiometric assay of calcitonin. J Clin Endocrinol Metab. 1994;78(1):114–120. doi:10.1210/jcem.78.1.7904610
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.