Last Updated on October 31, 2026
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If you have been treated for differentiated thyroid cancer (the papillary and follicular types that make up the vast majority of cases), one blood test probably keeps appearing on your lab orders: thyroglobulin, usually shortened to Tg.
This guide explains what Tg actually measures, why the same number can mean different things for different people, what can throw the result off, and how the 2025 American Thyroid Association (ATA) guidelines have changed how long many people need to keep testing.
What is thyroglobulin?
Thyroglobulin is a large protein your thyroid uses as raw material for making thyroid hormones. Crucially, it is made only by thyroid cells, both normal and cancerous. In someone with an intact thyroid, a little Tg always leaks into the bloodstream, roughly in proportion to the size of the gland.
Once the thyroid has been removed, that background supply largely disappears, so Tg should be undetectable or very low. If Tg is present or rising, it suggests thyroid cells are still alive somewhere in the body. Those cells may be harmless leftover tissue or cancer, which is why Tg is a monitoring tool rather than a diagnosis. Importantly, a Tg test on its own cannot diagnose cancer.
Many other cancers lack a blood marker this specific, and the ATA highlights Tg as a major advantage for detecting thyroid cancer recurrence early.
Your "good" number depends on your treatment
This is the single most important thing to understand about Tg. The amount of thyroid tissue left in your body after treatment sets the baseline, so the same result can be excellent for one person and worth a closer look for another.
Tg is measured in nanograms per milliliter (ng/mL). Some laboratories, including many in Europe, report it in micrograms per liter (µg/L), which gives exactly the same number.
The 2025 guidelines made the bar noticeably more relaxed for people who had a total thyroidectomy without radioactive iodine, a group that has grown as fewer low-risk patients receive RAI. The ATA’s patient summary explains that in this group, initial Tg levels below 2.5 ng/mL did not appear to be linked to higher recurrence risk. The guidelines also state that Tg predicts outcomes better in people who did receive RAI.
That 2.5 ng/mL cutoff is still being tested in real-world practice. One 2026 study in the journal Endocrine points out that the new threshold rests mainly on expert consensus rather than strong prospective data. Separately, a small study reported by Healio in October 2026 followed 205 adults treated without RAI and found three recurrences among people classed as excellent responders under the 2.5 ng/mL cutoff, all in higher-than-low risk groups. These are early findings, but they explain why doctors pair Tg results with neck ultrasound rather than relying on the blood test alone.
There's a catch though... Antibodies!
Some people’s immune systems make antibodies against thyroglobulin (TgAb). They are common: an estimated 15% to 30% of people with thyroid cancer have them. These antibodies can bind to Tg in the blood sample and hide it from standard lab tests, producing a falsely low or even undetectable result. Mayo Clinic Laboratories notes that when antibodies are present, standard immunoassays may underestimate Tg by up to 60%.
For this reason, the 2025 ATA guidelines recommend that antibodies be measured with every Tg test. If you do have antibodies, your Tg number on its own is not fully trustworthy, and your care team will lean on two other tools instead:
- The antibody trend. Antibody levels that are stable or falling over time are generally reassuring, while rising levels can be an early warning sign. These comparisons are only meaningful when the same antibody test is used each time.
- Imaging. For people with antibodies, the guidelines name imaging, mainly neck ultrasound, as the primary monitoring tool. Specialized mass spectrometry Tg tests exist, but the ATA notes they have limited sensitivity and should not be relied on alone.
Why the trend matters more than one result
A single Tg value is a snapshot. What your doctor really watches is the pattern across several tests. Change over time is more important than any one result. Three things affect how useful that pattern is.
- The lab method. Different Tg tests can give noticeably different numbers for the same blood sample, and Mayo Clinic Laboratories states that results from different methods cannot be directly compared. Switching labs can create a jump that reflects the test, not your body.
- Your TSH level at the time. TSH (thyroid-stimulating hormone) pushes any remaining thyroid cells to release more Tg. A “stimulated” Tg, measured after a TSH injection or after stopping thyroid hormone, runs higher than an ordinary on-treatment Tg, so the two should not be compared directly.
- How fast it changes. A steady, slowly drifting Tg is a different situation from one that is climbing quickly. Research has linked the Tg doubling time, meaning how long it takes for the level to double, to long-term outcomes in papillary thyroid cancer.
Before your next Tg blood draw
- Pause biotin if your lab asks you to. Biotin (vitamin B7), common in hair, skin, and nail supplements, can interfere with some lab tests. Some labs may ask patients to avoid biotin-containing supplements for 12 hours before a Tg blood draw. Follow your own lab’s instructions and mention any supplements you take.
- Stick with the same lab when possible. This keeps your results comparable over the years.
- Ask two quick questions. Were antibodies measured alongside Tg? And was this an on-treatment or a stimulated result?
- Keep your own record. Writing down each value, its date, the lab, and your antibody result makes trends much easier to spot.
How often and for how long?
According to the 2025 ATA guidelines, Tg is usually checked every 6 to 12 months during early follow-up, with more frequent testing for people at higher risk. A neck ultrasound is typically done 6 to 12 months after initial treatment, with later scans timed according to your risk.
The bigger news is that, for the first time, the guidelines describe when monitoring can stop. For people with low-risk cancer who keep an excellent response after a total thyroidectomy (with or without RAI), routine neck ultrasounds can be dropped after 5 to 8 years and Tg monitoring after 10 to 15 years. In between, blood tests every 1 to 2 years are usually enough. At that point, these patients are considered to be in “complete remission,” a term not previously used in thyroid cancer care.
Complete remission does not guarantee the cancer will never return, and the ATA is clear about that. But it can bring real relief, both emotionally and financially, and any decision to scale back should be made together with your doctor.
What if your Tg goes up?
A detectable or rising Tg is not an automatic recurrence. Mayo Clinic Laboratories notes that when Tg sits just above 1 ng/mL in someone with no thyroid left, the chance that it reflects residual or recurrent disease is modest, at roughly 3% to 25%. That probability rises considerably at higher levels.
In practice, a rising Tg usually leads first to a neck ultrasound. If the neck looks clear but Tg or antibody levels keep climbing, the guidelines recommend further imaging to check the lungs and bones, which are common sites of spread. Many people placed in the “indeterminate” category eventually turn out to be doing well, especially when their initial risk was low.
Key takeaways
- Thyroglobulin is a protein made only by thyroid cells, so after thyroid removal it works as a cancer marker.
- What counts as a “good” Tg level depends on whether you had radioactive iodine, a total thyroidectomy alone, or a lobectomy.
- Thyroglobulin antibodies can make Tg results falsely low, which is why antibodies should be checked with every Tg test.
- The direction of your results over time matters more than any single value, and comparisons only work when the same lab method is used.
- Under the 2025 ATA guidelines, many low-risk patients with a sustained excellent response can eventually stop routine monitoring.
