Thyroglobulin Unit Converter
Thyroglobulin Unit Converter
ng/mL and µg/L are the same number — convert between them, and then read the result the way a thyroglobulin has to be read: as a tumour marker for thyroid cancer follow-up, always alongside the thyroglobulin antibody, and always on one laboratory's assay.
Thyroglobulin converter
ng/mL = µg/LThyroglobulin 0.8 ng/mL, on levothyroxine after total thyroidectomy and radioiodine ablation
Why there is nothing to calculate
a nanogram in a millilitre and a microgram in a litre are the same concentration
and there is no molar unit: thyroglobulin is a ~660 kDa dimeric glycoprotein reported against a standard
- ng/mL = µg/L
- multiply by one. A thyroglobulin of 0.8 ng/mL is 0.8 µg/L. If two reports on the same patient differ, it is not the units
- no nmol/L or pmol/L
- thyroglobulin is a large, variably glycosylated dimer, so a mass concentration is the only meaningful report. Any molar figure for thyroglobulin has been invented
- CRM-457
- the international reference preparation most immunoassays are standardised against. Standardisation to a common material has narrowed the differences between methods but has not abolished them — results from different assays on the same serum can still differ substantially
- functional sensitivity
- the lowest concentration a method can reproduce, typically 0.1 ng/mL for a second-generation assay and around 1 ng/mL for an older one. It matters because the decision thresholds in cancer follow-up sit below 1 ng/mL
Worked example
Thyroglobulin 0.8 ng/mL, on levothyroxine after total thyroidectomy and radioiodine ablation
0.8 ng/mL = 0.80 µg/L — identical, and that is the whole of the arithmetic
Now the part that matters. In a patient with an intact thyroid, 0.8 ng/mL is unremarkable and says nothing about cancer; in this patient, who has no thyroid left, it is an ATA indeterminate response — detectable but below 1 ng/mL on suppressive therapy
Two questions have to be answered before that reading stands. Was the thyroglobulin antibody measured on the same sample, and was it negative? A positive antibody can suppress an immunometric thyroglobulin by 60% or more, so a true 3 ng/mL can be reported as 0.8
And was the previous value measured by the same method in the same laboratory? A change from 0.4 to 0.8 across two different assays may be no change at all
The trend on one method, with a negative antibody, is the measurement. A single number in isolation is not
The same number means different things in different patients
| Patient | How a thyroglobulin is read | Figures |
|---|---|---|
| Intact thyroid | Not a cancer test at all. Thyroglobulin is made by normal follicular tissue, and it is raised in goitre, thyroiditis, Graves’ disease, nodular disease and iodine deficiency. It cannot distinguish benign from malignant and must not be used to investigate a nodule | Mayo publish ≤33 ng/mL for patients with an intact thyroid, and state explicitly that this interval does not apply after surgery for thyroid cancer |
| After total thyroidectomy and radioiodine ablation | The setting the test exists for. With no thyroid tissue left, any thyroglobulin comes from residual or recurrent differentiated carcinoma, so the marker becomes interpretable and the thresholds fall two orders of magnitude | ATA 2015: excellent response — suppressed Tg <0.2 ng/mL or stimulated Tg <1 ng/mL. Indeterminate — detectable but suppressed Tg <1 or stimulated 10 ng/mL, or rising antibody |
| Lobectomy or an unablated remnant | The remaining normal tissue makes thyroglobulin, so the ATA thresholds above do not apply and only the trend over time is informative | No numerical threshold; interpretation is by trend on one assay |
| Congenital absence of thyroglobulin, or a neonatal work-up | An undetectable thyroglobulin has a different meaning entirely — it is used in the investigation of congenital hypothyroidism and of factitious thyrotoxicosis, where an undetectable Tg alongside a raised T4 points to exogenous hormone | Undetectable is the abnormal finding here |
Thyroglobulin antibody interference — why the two are always measured together
| Assay type | Effect of a positive thyroglobulin antibody | What to do |
|---|---|---|
| Immunometric (sandwich) assay — most modern automated methods | Antibody blocks the epitopes the assay needs, so the result is falsely low. Mayo quote underestimation of up to 60% | A low or undetectable thyroglobulin in an antibody-positive patient cannot be taken as reassurance |
| Competitive radioimmunoassay | Less vulnerable to negative interference, and can read falsely high | Sometimes used deliberately in antibody-positive patients, at the cost of poorer sensitivity |
| Thyroglobulin by LC-MS/MS | Measures a proteolytic peptide rather than an intact epitope, so antibody interference is largely avoided | The usual route to an interpretable thyroglobulin in an antibody-positive patient |
| The antibody itself, measured serially | Becomes a surrogate marker: a falling titre after treatment is reassuring, a rising one is an ATA criterion for a biochemical incomplete response | Measure the antibody on every sample, by the same method, and trend it alongside the thyroglobulin |
Why two laboratories disagree about the same serum
| Source of difference | Consequence |
|---|---|
| Different antibody pairs and epitope specificity between manufacturers, despite common standardisation to CRM-457 | The same serum can give substantially different values on two platforms — enough to move a patient across the 1 ng/mL ATA threshold |
| Different functional sensitivity | A second-generation assay reports 0.15 ng/mL where an older one reports “<1"; a change of method can look like a rise or a fall that never happened |
| Heterophile antibodies, and the high-dose hook effect at very high concentrations | Spuriously raised or, rarely, spuriously low results |
| Antibody status changing over time | A patient who becomes antibody-positive acquires a downward bias mid-series |
A tumour marker, not a thyroid function test
The unit conversion on this page is a formality. Nanograms per millilitre and micrograms per litre are the same concentration written two ways, so a thyroglobulin of 0.8 ng/mL is 0.8 µg/L and nothing needs multiplying. There is no molar unit either: thyroglobulin is a dimeric glycoprotein of about 660 kilodaltons, variably glycosylated, and is reported as a mass concentration against a reference preparation rather than as an amount of substance. Any thyroglobulin quoted in nanomoles per litre has been invented somewhere along the way.
What the page is really for is the interpretation, because thyroglobulin is the thyroid test most often misused. It is not a thyroid function test and it says nothing about whether a patient is hypothyroid or thyrotoxic. It is a tumour marker for the follow-up of differentiated thyroid carcinoma, and it is only interpretable once the thyroid has been removed and any remnant ablated. Normal follicular tissue makes thyroglobulin, so in a patient with an intact gland the concentration reflects thyroid mass and activity: it is raised in goitre, in thyroiditis, in Graves’ disease and in iodine deficiency, it does not distinguish benign from malignant, and it has no place in the investigation of a thyroid nodule. After total thyroidectomy and radioiodine ablation the picture inverts — there should be no source of thyroglobulin at all, so any measurable concentration points to residual or recurrent disease, and the thresholds that matter fall two orders of magnitude below the intact-thyroid reference interval, to fractions of a nanogram per millilitre.
Two caveats are not optional. The first is that thyroglobulin antibodies interfere with the assay and characteristically make it read falsely low: in the immunometric methods most laboratories use, a positive antibody blocks the epitopes the assay depends on, and the thyroglobulin can be underestimated by 60% or more. Roughly a quarter of patients with differentiated thyroid cancer are antibody-positive. A reassuringly undetectable thyroglobulin in such a patient is not reassurance at all, which is why the antibody is measured on the same sample every time, why a rising antibody titre is itself an ATA criterion for an incomplete biochemical response, and why measurement by mass spectrometry is used when an interpretable number is needed in an antibody-positive patient.
The second is that thyroglobulin results are not comparable between assays or between laboratories. Common standardisation to the CRM-457 reference material has narrowed the spread without closing it, functional sensitivities differ by an order of magnitude between generations of assay, and heterophile antibodies and the hook effect add their own errors. A patient should therefore be trended on one method in one laboratory, with the method recorded; a change of assay starts a new series and an apparent rise across two platforms may be no change at all.
Frequently asked questions
Is a thyroglobulin of 1 ng/mL the same as 1 µg/L?
Yes — exactly the same concentration. A nanogram in a millilitre and a microgram in a litre are numerically identical, so no conversion is needed. There is no molar unit for thyroglobulin because it is a 660 kDa variably glycosylated glycoprotein reported against a reference preparation, so a value in nmol/L or pmol/L is not a real thyroglobulin unit.
Why is thyroglobulin always measured with the thyroglobulin antibody?
Because the antibody interferes with the assay, and in the immunometric methods most laboratories use it makes the result falsely low — Mayo quote underestimation of up to 60%. Around a quarter of patients with differentiated thyroid cancer are antibody-positive, so an undetectable thyroglobulin in an antibody-positive patient cannot be taken as evidence of no disease. The antibody is measured on the same specimen every time; a rising titre is itself an ATA criterion for a biochemical incomplete response, and measurement by mass spectrometry is the usual way to get an interpretable number when antibodies are present.
Can thyroglobulin be used to look for thyroid cancer in someone who still has a thyroid?
No. Normal follicular tissue makes thyroglobulin, so the concentration reflects thyroid mass and activity and is raised in goitre, thyroiditis, Graves’ disease and iodine deficiency. It does not distinguish benign from malignant and has no role in the investigation of a thyroid nodule. Thyroglobulin becomes interpretable as a tumour marker only after thyroidectomy and ablation, when there should be no normal source of it left.
Why does this page show no reference interval?
Because no single interval fits both populations that have their thyroglobulin measured. Mayo publish ≤33 ng/mL for patients with an intact thyroid and say in the same document that it does not apply to patients operated on for thyroid cancer — who are the patients the test is actually for, and whose thresholds are two orders of magnitude lower, at 0.2 to 1 ng/mL. Printing “within range” over a thyroglobulin of 20 ng/mL in an ablated patient would be reassurance about biochemical evidence of persistent disease. Both sets of figures are in the tables, each labelled with the population it belongs to.
Can thyroglobulin results from two different laboratories be compared?
Not safely. Despite common standardisation against the CRM-457 reference material, different antibody pairs and epitope specificities give different values on the same serum, functional sensitivity varies by an order of magnitude between assay generations, and heterophile antibodies and the hook effect add further error. Trend a patient on one method in one laboratory, record which method it was, and treat a change of assay as the start of a new series.
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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. doi:10.1089/thy.2015.0020 — response-to-therapy definitions: excellent response suppressed Tg <0.2 ng/mL or stimulated Tg 10 ng/mL, or rising anti-Tg antibody.
- Mayo Clinic Laboratories. Test ID: HTG2 — Thyroglobulin, Tumor Marker, Serum. Immunoenzymatic assay. Reference value ≤33 ng/mL, with the statement that thyroglobulin reference intervals are for patients with an intact thyroid and not for patients who have had surgery for thyroid cancer; thyroglobulin antibody <1.8 IU/mL. Notes underestimation of Tg by up to 60% in immunoassays when antibodies are present.
- Netzel BC, Grebe SKG, Carranza Leon BG, et al. Thyroglobulin (Tg) testing revisited: Tg assays, TgAb assays, and correlation of results with clinical outcomes. J Clin Endocrinol Metab. 2015;100(8):E1074–E1083.
- 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.
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.