Calcitonin

What Calcitonin actually measures — the analyte, and the assay

This page is for a reader holding a calcitonin result, and nothing on it is a reason to obtain one. Ordered in people without symptoms, this marker's documented behavior — spurious elevations from antibodies in the sample, and reference values that depend on which assay ran — produces investigations far more often than it produces findings. The sections below explain the number, not the person.

Calcitonin is a 32-amino-acid peptide from the thyroid's parafollicular C cells, spliced from the same CALCA transcript that yields calcitonin gene-related peptide in neural tissue. Its role in adult human calcium handling is minor: people without a thyroid do not become hypercalcemic. It is measured because C cells make it, not because it does much.

The assay generation changed and took the reference range with it. Older radioimmunoassays detected calcitonin together with its precursors and cross-reacting fragments. Modern two-site immunochemiluminometric assays detect the monomer alone, and the printed normal values fell by roughly an order of magnitude. A result from the 1990s and a result from today are not the same measurement.

Even among current assays the disagreement is severe, and it has been quantified. In 375 clinically euthyroid subjects, the proportion of samples in which no calcitonin was detected at all ranged from 56% on one assay to 88% on another d'Herbomez 2007. Same people, same sera: on one platform nearly nine in ten healthy adults are undetectable and on another barely five in ten are.

Calcitonin: what changes the blood, and what only changes the reading

What changes the calcitonin in your body:

1. Sex, because C-cell mass differs. Men run higher, which is why this page prints two limits and why stimulated testing uses gender-specific cut-off levels rather than one Niederle 2018 d'Herbomez 2007.
2. Smoking and age, both examined in the same reference-range work d'Herbomez 2007.
3. Reduced renal clearance, which raises the circulating concentration of a small peptide without any change in production.
4. Chronic hypergastrinemia, including that produced by sustained acid suppression, since gastrin is a physiological C-cell secretagogue.
5. Autoimmune thyroiditis and other thyroid disease, which alter the C-cell environment.
6. Neuroendocrine tissue elsewhere, since C cells are not the only cells capable of making the peptide.
7. A stimulation test. Calcium- or pentagastrin-stimulated calcitonin is a deliberately provoked number with its own thresholds and is not comparable with a resting value Niederle 2018.

What changes only the reading — and on this marker the false-high mechanism has led to real intervention:

1. Heterophilic antibodies. Antibodies in the sample that bind the animal immunoglobulins used in the assay bridge capture to detection and produce signal from nothing. Falsely high serum calcitonin from this mechanism was documented in 2006 Papapetrou 2006, and spurious hypercalcitoninemia has been described specifically in patients under investigation for thyroid nodules Giovanella 2011 — that is, in exactly the group in whom a raised result triggers action.
2. Rheumatoid factor, which is an anti-IgG antibody and therefore a purpose-built bridge. A raised rheumatoid factor produced both a falsely elevated calcitonin and a falsely elevated thyroglobulin in one patient whose carcinoma had not relapsed Lupoli 2019.
3. Several interferences at once, which has been reported in a single patient with a falsely elevated calcitonin, and which is why one blocking step is not always enough Unal 2023.
4. Which assay. The 56%-to-88% spread above is not a rounding difference; it is the difference between a result and no result d'Herbomez 2007.
5. Antigen excess at very high concentrations, which can saturate both antibodies separately on a one-step assay and depress the reported value.
6. Delayed separation or a warm sample, since the peptide is not stable indefinitely.

Reference interval or decision threshold — which kind of number Calcitonin is

These are reference intervals, they are sex-specific, and on many platforms most of the reference population sits below the detection limit. That last point deserves attention: when 88% of healthy subjects are undetectable on an assay d'Herbomez 2007, normal does not describe a central band. It describes an absence.

Which means a number where there was usually none is what draws attention — and it is also the situation in which an antibody artifact is most likely to be mistaken for a finding Papapetrou 2006 Giovanella 2011.

The stimulated test is a decision threshold and a different test. Calcium-stimulated calcitonin was proposed with clinically relevant gender-specific cut-off levels for the specific purpose of improving on the resting value Niederle 2018. Those cut-offs cannot be applied to a basal result.

There is no optimal calcitonin and no direction to move it in. Nothing on this site treats it as a modifiable number.

How you would know your Calcitonin was wrong — and when to redraw

Before repeating the biology, repeat the chemistry. Calcitonin's circulating half-life is short, but the question a surprising result raises is almost never about kinetics. It is whether the assay was reporting an antibody.

How you would know the value was an artifact:

• Ask the laboratory to re-run the sample on a different manufacturer's assay. An interference is usually specific to one antibody pair; a real peptide is present on both d'Herbomez 2007.
• Ask for a heterophilic blocking tube and a serial dilution. A true analyte dilutes proportionally and an antibody bridge does not. This is the sequence that identified the published cases Papapetrou 2006 Giovanella 2011 Unal 2023.
• Check a Rheumatoid Factor. It is cheap, and it is a documented cause of exactly this result Lupoli 2019.
• Check kidney function on the CMP, which explains a modest stable elevation that no amount of imaging will.
• Then stop, and take it to the clinician who ordered it. Whether a confirmed elevation warrants stimulated testing, imaging or nothing at all is a specialist judgment that depends on the history, the examination and the ultrasound — and it is not a judgment a webpage should make about anybody.

If a series is being followed, the platform must not change mid-series, for the same reason two assays disagreed about whether healthy people have any calcitonin at all d'Herbomez 2007.

What Calcitonin cannot tell you

It cannot separate C-cell hyperplasia from anything else, and no single value distinguishes a benign cause from a serious one. That is why stimulated testing was developed and why interpretation is a specialist task Niederle 2018.

It cannot be trusted in isolation when it is unexpected. With at least three separate documented mechanisms producing falsely high results Papapetrou 2006 Giovanella 2011 Unal 2023 Lupoli 2019, an isolated elevation in someone with no other abnormality has a substantial prior probability of being the assay.

It cannot be compared with a value from another laboratory or another decade. The assay generation changed the reference range, and current assays disagree with each other d'Herbomez 2007.

It is not a screening test, and routine surveillance in people taking drugs carrying a rodent C-cell warning shares the same problem. A test whose false-positive mechanisms are this well documented, applied to a population in which the target condition is rare, returns mostly artifacts — each of which costs somebody an ultrasound, a biopsy or an operation.

The wrong inference readers actually draw is that a detectable calcitonin is itself abnormal. On a sensitive assay a substantial minority of entirely healthy people have one d'Herbomez 2007, and the question is never the presence of the peptide but whether the value is real, reproducible and rising.

Sources read for these sections

• d'Herbomez M, et al. Reference range of serum calcitonin levels in humans: influence of calcitonin assays, sex, age, and cigarette smoking. European Journal of Endocrinology 2007 · PMID 18057382
• Niederle MB, et al. Calcium-stimulated calcitonin - The new standard in the diagnosis of thyroid C-cell disease - clinically relevant gender-specific cut-off levels for an old test. Biochemia Medica 2018 · PMID 30429678
• Papapetrou PD, et al. Heterophilic antibodies causing falsely high serum calcitonin values. Journal of Endocrinological Investigation 2006 · PMID 17185902
• Giovanella L, et al. Spurious hypercalcitoninemia and heterophilic antibodies in patients with thyroid nodules. Head and Neck 2011 · PMID 20848426
• Unal MC, et al. Multiple immunoassay interference in a patient with falsely elevated calcitonin. Archives of Endocrinology and Metabolism 2023 · PMID 37988668
• Lupoli GA, et al. Falsely elevated thyroglobulin and calcitonin due to rheumatoid factor in non-relapsing thyroid carcinoma. Medicine (Baltimore) 2019 · PMID 30702570