What this test measures
Thyroglobulin (Tg) is the large protein scaffold inside thyroid follicles on which thyroid hormones (T3 and T4) are built. It is made only by thyroid tissue — normal or cancerous. After complete removal of the thyroid (total thyroidectomy) and any residual thyroid tissue (radioactive iodine ablation), serum thyroglobulin should be undetectable.
The serum assay measures Tg by immunoassay. Sensitive "second-generation" assays now go down to about 0.1 ng/mL — much better than older assays that could not distinguish "low" from "absent" disease. Anti-thyroglobulin antibodies (anti-Tg) must always be measured alongside, because they interfere with the Tg assay and can give falsely low results — even a single negative or rising anti-Tg trend can itself be a surrogate marker of disease.
Why it matters
Differentiated thyroid cancer (papillary and follicular) is one of the commonest endocrine cancers in India, with rising incidence especially in young women. After treatment (surgery ± radioactive iodine), Tg is the principal long-term tumour marker — a rising Tg is often the earliest signal of recurrence, months before imaging shows disease.
For maximum sensitivity, Tg is sometimes measured after TSH stimulation (either thyroxine withdrawal or recombinant TSH injection) — TSH stimulates any residual thyroid cells to release Tg. ATA-guided risk stratification uses Tg and anti-Tg trends to guide imaging and to decide between observation and additional therapy.
How to prepare
No fasting required. Continue levothyroxine as usual unless a stimulated Tg is being done — your endocrinologist will give specific timing. Stop biotin for 48–72 hours. The lab must run anti-Tg on the same sample. Tell the lab about your thyroid cancer history.
Markers & reference ranges
Reference ranges below are typical adult values. Your lab's reported range may differ slightly based on the assay platform and patient demographics — always read your report against the range printed on it.
| Marker | Normal range | If low | If high |
|---|---|---|---|
| Thyroglobulin (ng/mL)[1][2] | Intact thyroid (non-cancer): 3 – 40 · After total thyroidectomy + radioiodine: < 0.1–0.2 (undetectable is goal) | Undetectable Tg after total thyroidectomy + radioiodine is the target — strongly suggests no residual or recurrent disease. Low Tg in a thyroid cancer patient with positive anti-Tg may be falsely low — interpret with the anti-Tg trend. | Detectable Tg in a thyroidectomised patient suggests residual or recurrent thyroid tissue / cancer — needs imaging (neck USG, whole-body iodine scan, sometimes PET-CT). Any rise from previous baseline is concerning even if absolute value is low. Tg above the reference range in someone with an intact thyroid is non-specific — can occur with thyroiditis, goitre, or thyroid cancer. |
| Anti-Thyroglobulin (companion) (IU/mL) | < 4.0 IU/mL (assay-dependent) | Negative anti-Tg means the Tg assay can be trusted. | Positive anti-Tg makes Tg unreliable — track the anti-Tg trend as a surrogate marker (a rise is concerning). |
Thyroglobulin — post-thyroidectomy interpretation
| Tg (ng/mL) | Anti-Tg | Interpretation |
|---|---|---|
| Undetectable (< 0.1) | Negative | Excellent response — no disease evidence |
| Detectable (0.1 – 1.0) | Negative | Indeterminate / biochemical incomplete — close follow-up |
| > 1.0 or rising | Negative | Biochemical recurrence — imaging indicated |
| Any value | Positive | Tg unreliable — track anti-Tg trend |
| Stimulated Tg > 2 | Negative | Residual disease likely — imaging |
Frequently asked questions
I had thyroid cancer surgery years ago — why is Tg still being checked?
Thyroglobulin is the long-term tumour marker for differentiated thyroid cancer. It is the most sensitive way to detect recurrence — often years before imaging shows disease. Lifelong monitoring is standard, although intervals stretch out as time passes with undetectable Tg.
Why does the lab also test anti-Tg?
Anti-Tg antibodies interfere with the Tg assay and can give falsely low readings. Your doctor needs both numbers to interpret the result. If anti-Tg is positive, the antibody trend itself is followed as a surrogate marker.
What is a "stimulated" thyroglobulin test?
A way to make the assay more sensitive. TSH is allowed to rise — either by stopping levothyroxine for several weeks or by injecting recombinant TSH (Thyrogen). The raised TSH stimulates any residual thyroid cells to release Tg, making small amounts of disease detectable.
My Tg is detectable but I feel fine — does that mean recurrence?
A detectable Tg in a thyroidectomised patient is always taken seriously — but a single low detectable value (0.1–1 ng/mL) often represents indeterminate biochemical incomplete response and is monitored rather than treated immediately. A rising trend is more concerning than any single value.
Does Tg help in people who still have their thyroid?
Less so — a thyroid that is still in place produces Tg normally, and the test cannot distinguish benign tissue from cancer. Its main role is post-operative follow-up.
How often is Tg repeated after thyroid cancer surgery?
Typically every 6 months for the first 2–3 years, then annually if Tg remains undetectable. Higher-risk patients are checked more often.
Will biotin affect the result?
Yes — high-dose biotin interferes with most Tg immunoassays. Stop 48–72 hours before the test.
Related Hormones / Endocrine tests
Tests commonly ordered alongside THYROGLOBULIN (TG), or that help interpret an unexpected result.
Sources & references
- American Thyroid Association — Thyroid Cancer Management Guideline · accessed 2026-05-30T00:00:00.000Z
- NIH MedlinePlus — Thyroglobulin Test · accessed 2026-05-30T00:00:00.000Z
- NCBI StatPearls — Thyroglobulin · accessed 2026-05-30T00:00:00.000Z
- Mayo Clinic Laboratories — Thyroglobulin · accessed 2026-05-30T00:00:00.000Z
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