Dear Dr Ayati, I think Anti Tg is not a tumor marker for thyroid cancer. But hTg is a tumor marker, we must alert about its false negative results which may occur in recurrence or metastasis situation. We expect increase in hTg level in TC recurrence or metastasis, but in presence of AntiTg, it may hTg bind to Anti Tg and does not react with the antibody in immunoassay kit , so we encounter to false negative results.
In the presence of anti-Tg, Tg may be exceptional low in IMA assay if there is persistent or recurrent tumour. Therefore, Tg may not be reliable as tumour marker in this situation. But, the titre of the anti-Tg (antibody) is closely related to the titre of Tg (antigen). The reducing trend of antiTg (antibody) after treatment then may reflect the decreased of the stimulating antigen which is the Tg. Hence, it can be used as surrogate marker for disease monitoring. But it is not a real tumour marker as there is not actual cutoff level to indicating disease and its level is highly fluctuating depending on the presence ofTg and other factors that may influence seroconversion.
It is not codified, but in some cases the detection of elevated AbTg levels can anticipate the increase of Tg levels.
If anti tg level is rising, yes.
In some patients with undifferentiated PTC, yes. (eg. at first presentation, WBIS showed LN or distant metastasis with ,low tg level. So, in this patient follow up: tg level is not reliable.)
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