Anti-Thyroglobulin Antibody Unit Converter
Anti-Thyroglobulin Antibody Unit Converter
Convert thyroglobulin antibody between IU/mL, kIU/L and IU/L — the first two are the same number — and read it for what it mostly is: the companion test that decides whether a thyroglobulin result can be believed.
Anti-Thyroglobulin Antibody converter
IU/mL = kIU/LThyroglobulin antibody 20 IU/mL, reported alongside a thyroglobulin of 0.4 ng/mL after thyroidectomy and ablation
The units are interchangeable; the results are not
1 IU/mL = 1,000 IU/L
and no conversion exists between one manufacturer’s IU/mL and another’s
- IU/mL = kIU/L = U/mL
- the same number, so 20 IU/mL is 20 kIU/L. Manufacturers writing U/mL mean the same arbitrary unit
- IU/L
- the IU/mL figure × 1,000, so 20 IU/mL is 20,000 IU/L
- an arbitrary unit
- no mass or molar equivalent exists. The assay measures binding to a thyroglobulin preparation, calibrated against the manufacturer's own standard, so results are not transferable between methods
- why the assay is even more variable than most
- thyroglobulin antibodies are polyclonal and recognise different epitopes in different patients, and each manufacturer's thyroglobulin antigen presents a different subset of them. Between-method discordance is large — the same serum can be positive on one platform and negative on another
- what it is measured with
- almost always thyroglobulin, on the same specimen. The two results are read together, not separately
Worked example
Thyroglobulin antibody 20 IU/mL, reported alongside a thyroglobulin of 0.4 ng/mL after thyroidectomy and ablation
20 IU/mL = 20.0 kIU/L = 20.0 U/mL = 20,000 IU/L
Against Mayo's reference value of <1.8 IU/mL this is positive — so the thyroglobulin reported beside it cannot be taken at face value
In an immunometric thyroglobulin assay a positive antibody blocks the epitopes the method needs and the result reads falsely low, by up to 60%. A reassuring 0.4 ng/mL could correspond to a true value several times higher
The next step is a thyroglobulin measured by LC-MS/MS, which is largely immune to the interference, and serial measurement of the antibody itself: a falling titre after treatment is reassuring, a rising one is an ATA criterion for a biochemical incomplete response
And as with any antibody in arbitrary units, the trend only exists within one assay. A titre of 20 on this platform and 60 on another are not two data points in a series
What a thyroglobulin antibody is actually for
| Use | Standing |
|---|---|
| Interpreting a thyroglobulin in thyroid cancer follow-up | The main use. A positive antibody makes an immunometric thyroglobulin read falsely low — by up to 60% — so the antibody is measured on every specimen alongside the thyroglobulin, and a low thyroglobulin in an antibody-positive patient is not reassurance |
| Serial antibody titre as a surrogate tumour marker | Useful in antibody-positive patients, where the thyroglobulin itself is unreliable: a falling titre after treatment is reassuring, and a rising titre is an ATA 2015 criterion for a biochemical incomplete response |
| Diagnosing autoimmune thyroiditis | Adds little. Anti-TPO is the more sensitive and more useful marker, and in NHANES III thyroglobulin antibodies without thyroid peroxidase antibodies were not associated with abnormal thyroid function. Measuring anti-Tg alone for this purpose is not worth doing |
| Monitoring thyroid function in autoimmune thyroid disease | No role. TSH is the monitoring test; the titre does not track thyroid status |
| Investigating a thyroid nodule | No role, either alone or with thyroglobulin |
Figures, and whose they are
| Item | Figure | Source |
|---|---|---|
| Mayo Clinic Laboratories reference value | Thyroglobulin antibody <1.8 IU/mL, reported with their thyroglobulin tumour-marker assay | Mayo test HTG2 |
| Underestimation of thyroglobulin in immunoassay when antibodies are present | Up to 60% | Mayo test HTG2 interpretive comment |
| Thyroglobulin antibodies in the disease-free US population | About 10%; positivity without anti-TPO was not associated with abnormal TSH | NHANES III |
| Thyroglobulin antibody prevalence in differentiated thyroid cancer | Roughly a quarter of patients — too common for the interference to be an edge case | Spencer, and the ATA 2015 guideline |
| Comparability between manufacturers’ assays | Poor. The same serum can be positive on one platform and negative on another | Assay-specific; this is why no interval is printed on this page |
The companion test that decides whether a thyroglobulin can be believed
Thyroglobulin antibodies are reported in international units per millilitre, kilo-international units per litre or units per millilitre, and those three are the same number — 20 IU/mL is 20 kIU/L. International units per litre, where used, are that figure multiplied by a thousand. As with the thyroid peroxidase antibody there is no mass or molar unit, because the assay measures binding to a thyroglobulin preparation calibrated against the manufacturer’s own standard rather than a defined quantity of a defined molecule.
And as with anti-TPO, the critical point is that results are not comparable between assays or manufacturers. It is arguably worse for this antibody than for any other thyroid test: thyroglobulin antibodies are polyclonal and different patients recognise different epitopes, while each manufacturer’s thyroglobulin antigen presents a different subset of them. The same serum can be positive on one platform and negative on another. A numerical change therefore means nothing unless both results came from the same method in the same laboratory, and this page prints no reference interval under the answer for that reason — Mayo’s value of less than 1.8 IU/mL is in the table above, labelled as belonging to their assay.
The reason the antibody is measured at all, most of the time, is analytical rather than diagnostic. Thyroglobulin is the tumour marker used to follow differentiated thyroid carcinoma after thyroidectomy and ablation, and a positive thyroglobulin antibody interferes with it: in the immunometric assays most laboratories run, the antibody blocks the epitopes the method depends on and the thyroglobulin is reported falsely low, by up to 60%. Around a quarter of patients with differentiated thyroid cancer are antibody-positive, so this is routine rather than exceptional. The antibody is therefore measured on every specimen alongside the thyroglobulin, a low thyroglobulin in an antibody-positive patient is not taken as reassurance, and an interpretable result is usually obtained by measuring thyroglobulin by mass spectrometry instead. The titre itself then becomes a crude surrogate marker: falling after successful treatment, and rising as one of the ATA’s criteria for a biochemical incomplete response.
As a diagnostic test for autoimmune thyroid disease, the thyroglobulin antibody adds little. Anti-TPO is more sensitive and more informative, and in NHANES III thyroglobulin antibody positivity in the absence of thyroid peroxidase antibodies was not associated with abnormal thyroid function — it is present in about one person in ten of the disease-free population. Measuring it on its own to investigate suspected autoimmune thyroiditis is not worth doing, and it has no role in monitoring thyroid function, for which TSH is the test, or in the investigation of a thyroid nodule.
Frequently asked questions
Is 20 IU/mL of thyroglobulin antibody the same as 20 kIU/L?
Yes — IU/mL, kIU/L and U/mL are numerically identical for this analyte, so nothing needs converting between them. IU/L is the IU/mL figure multiplied by a thousand, making 20 IU/mL equal to 20,000 IU/L. There is no molar or mass unit, because the result is binding activity in arbitrary units against the manufacturer’s calibration.
Why is the thyroglobulin antibody measured with thyroglobulin?
Because a positive antibody makes the thyroglobulin assay unreliable, and characteristically falsely low: in the immunometric methods most laboratories use, the antibody blocks the epitopes the assay needs and the thyroglobulin can be underestimated by up to 60%. About a quarter of patients with differentiated thyroid cancer are antibody-positive, so the antibody is measured on every specimen alongside the thyroglobulin. A low thyroglobulin in an antibody-positive patient is not reassurance, and an interpretable result usually requires measuring thyroglobulin by mass spectrometry.
Can thyroglobulin antibody results from different laboratories be compared?
No, and this is one of the least comparable assays in thyroid practice. The antibodies are polyclonal and different patients recognise different epitopes, while each manufacturer’s antigen preparation presents a different subset of them — so the same serum can be positive on one platform and negative on another. Trend a patient on one assay in one laboratory and treat a change of method as the start of a new series.
Is a thyroglobulin antibody useful for diagnosing Hashimoto's thyroiditis?
It adds little once anti-TPO has been measured. Thyroid peroxidase antibody is the more sensitive and more useful marker of autoimmune thyroid disease, and in NHANES III thyroglobulin antibodies without thyroid peroxidase antibodies were not associated with abnormal thyroid function. About one person in ten of the disease-free population is thyroglobulin antibody-positive, so a positive result on its own is not a diagnosis.
Why does this page show no reference interval?
Because the cut-off belongs to the assay rather than to the patient, and the between-method discordance for this antibody is particularly large. Mayo report a reference value of less than 1.8 IU/mL with their thyroglobulin tumour-marker assay; other platforms publish quite different thresholds. That figure is in the table above, named with the assay it came from, rather than printed under the converted answer where it would invite comparison across methods.
Related calculators
References
- Haugen BR, Alexander EK, Bible KC, et al. 2015 American Thyroid Association Management Guidelines for Adult Patients with Thyroid Nodules and Differentiated Thyroid Cancer. Thyroid. 2016;26(1):1–133 — rising anti-Tg antibody as a criterion for biochemical incomplete response.
- Spencer CA. Clinical utility of thyroglobulin antibody (TgAb) measurements for patients with differentiated thyroid cancers (DTC). J Clin Endocrinol Metab. 2011;96(12):3615–3627.
- Mayo Clinic Laboratories. Test ID: HTG2 — Thyroglobulin, Tumor Marker, Serum. Thyroglobulin antibody reference value <1.8 IU/mL; thyroglobulin may be underestimated by up to 60% in immunoassays when antibodies are present.
- Hollowell JG, Staehling NW, Flanders WD, et al. Serum TSH, T4, and thyroid antibodies in the United States population (1988 to 1994): National Health and Nutrition Examination Survey (NHANES III). J Clin Endocrinol Metab. 2002;87(2):489–499.
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.