Thyroglobulin

A bit of a mouthful, but something that patients with thyroid cancer become quickly familiar with, not least because the doctors keep on mentioning it!

Thyroglobulin (or “Tg” to be kinder to the tongue) is a substance made by thyroid cells. This is unique as no other tissue or cell in the body is capable of making Tg.

Thyroid cells usually store Tg, but some of it leaks into the blood stream and can be measured in a blood sample. Patients with thyroid cancer who have been cured by surgery and radioiodine, have no detectable Tg in their blood stream.

If the cancer comes back, the Tg blood test becomes positive long before the cancer causes symptoms. So the Tg blood test can be used as a marker of thyroid cancer and it can pick up recurrence and give doctors the opportunity to offer early treatment.

In some patients the Tg blood test takes time to become negative after radioiodine and it may detectable at low levels for a year.

In some cases it can be detectable at low levels for a very long time. What is sometimes difficult to know is whether low levels of Tg in the blood are due to persistent thyroid cancer, or normal thyroid tissue that has not been removed by surgery and radioiodine.

Doctors then may use other tests to try and find out, or simply look for a trend in the levels of Tg over time.

The Tg blood test becomes much more sensitive when there are high levels of the hormone TSH in the circulation. TSH (stands for “thyroid stimulating hormone”) comes from the pituitary gland and the levels can be raised either by stopping the thyroxine treatment, or by injections of synthetic TSH (Thyrogen).

TSH seems to be able to “squeeze” Tg out of thyroid cells, so in some cases where there are microscopic amounts of thyroid cancer in the body, the Tg can be detected only after the blood levels of TSH are raised.

This is why we measure Tg every time patients come off thyroxine or after Thyrogen.

Sometimes patients with thyroid cancer have antibodies to Tg, which make it technically difficult to measure Tg accurately and then doctors have to se other means of monitoring the condition (for example scans).

Tg is not a marker for some of the rarer thyroid cancers like medullary thyroid cancer or lymphoma of the thyroid.

Tg is a useful tool in monitoring most patients with thyroid cancer. Sometimes however, patients and doctors (particularly if inexperienced) can get carried away in chasing the slightly raised Tg result with excessive investigations that usually generate a lot of anxiety and hardly ever yield any valuable information.

Tg monitoring is only part of how patients with thyroid cancer should be managed, and needs to be put in context of the individual case.

This is why these days such decisions are taken after careful consideration and discussion among the thyroid cancer experts in multi-disciplinary team meetings.

So, if your Tg result is negative it is reassuring news, but if it is detectable my advice is: don’t try to interpret the result yourself; ask your specialist to explain what that means in your individual case.

Petros Perros (Consultant Endocrinologist)
Joint Thyroid Cancer Clinic
Northern Centre for Cancer Care
Newcastle upon Tyne

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