What is the goal of RAI if the Tg is undectable?

[Justin1967]

If the Tg is undectable or negative is that is good as it gets?

Or is the Tg blood test not accurate enough?

My TSH is 0.4 and FT4 is 17

About Me
I had surgery on the 30th May and stayed 3 nights in hospital.  Results from pathology showed that the larger nodule was benign follicular adenoma ( 35mm diameter), while the smaller nodule 5mm was a papillary cariconoma.  Thus the procedure was converted to a  complete thyroidectomy.
The cancer was very small 5mm pap cancer with 2 other pap carcinoma on the other lobe, the largest being 3mm.

[GravesLady]

In patients with a known diagnosis of well differentiated thyroid cancer (most types of papillary and follicular disease), the serum thyroglobulin (Tg) is a useful ancillary marker of disease activity and provides information about the state and extent of residual functioning thyroid tissue in patients both on and off L-thyroxine suppression or after stimulation with recombinant TSH. Ideally, the thyroglobulin levels will be low or undetectable after treatment (usually surgery followed by radioactive iodine).  
However, there are certain pitfalls to the interpretation of thyroglobulin levels, which may include assay variability and the presence of thyroglobulin antibodies.

[Justin1967]

What do you mean by interpretation of thyroglobulin levels, which may include assay variability and the presence of thyroglobulin antibodies. What are thyroglobulin antibodies?

[utahmomma]

Did you have RAI already?  After a diagnosis of multi-focal papillary (or any type) carcinoma, RAI is recommended to remove any thyroid tissue which couldn't be surgically removed.

I just had my thyroglobulin levels run, again, a year after my RAI and they are 0.2.  If you *haven't* had RAI I would be shocked that yours are undetectable.  My sister needed three courses of RAI before her thyroglobulin levels were undetectable.

The note on my thyroglobulin test states:  "Thyroglobulin antibodies (anti-TG) are known to interfere with the measurement of thyroglobulin.  An assessment of autoantibody interference can be made by a recovery study.  A recovery of less than 80% suggests interference in the assay and the thyroglobulin value should be interpreted with caution"

Here's a good definition of the anti-TG test from Lab Tests Online http://labtestsonline.org/understanding/analytes/thyroglobulin/test.html

"A thyroglobulin antibody (TgAb) test may be ordered along with the thyroglobulin test. Thyroglobulin antibodies are proteins that the body’s immune system develops to attack thyroglobulin. These antibodies can develop at any time and when they are present, they interfere with the thyroglobulin test. Once they have developed, they will not go away and from that point forward, the TgAb test should be ordered with every thyroglobulin test."

Since TG tests are the standard tumor marker for most thyroid cancer patients it is important for your endo to know if your results can be trusted or are being interfered with by antibodies.

Clear as mud??  :-)

[Justin1967]

No I haven't had RAI.  I'm trying to find out if I really need it.

Thyroglobulin antibodies is 18 IU/ml (0-150) and my
thyroid Peroxidase is 7 IU/ml (0-150). My not sure what these mean?

my Tg  is <1 ug/l (<55)
and is says that the presence of anti-thyroglobulin antibodies may cause falsely low thyroglobulin  results, including undetectable thyroglobulin results.

[utahmomma]

Many endos and surgeons would say "yes" you need RAI after having multiple papillary carcinomas discovered.  I waited for three years after my surgery and, sure enough, my thyroglobulin levels and TSH levels started to climb indicating a recurrance.  The RAI confirmed it.  It's something you really need to discuss with your doctor.

GravesLady is the queen of labs - I'm not.  She can tell you whether 18 IU/ml would be significant enough to give a false TG reading.  However, I do know that no one who has had a TT without RAI should have a negative TG because there will still be thyroid tissue present, no matter how skilled the surgeon.  So my very uneducated guess would be that your TG results are being interfered with.

Just my $.02 worth

[Justin1967]

So did you wait three years before you had RAI?

[utahmomma]

I did because I only had a small (5mm), single papillary carcinoma.  I wouldn't have needed the RAI if my TG and TSH hadn't started climbing despite a stable dose of synthroid.  It indicated a recurrance of the cancer and the scans during the RAI confirmed it.  That's why getting an accurate TG reading is important.  

If I would have had more than a solitary carcinoma then RAI would have been indicated directly after the TT.