Tg and antiTg tests are not used to diagnose thyroid cancer, this test is only valuable for monitoring the disease after treatment for well-differentiated thyroid cancer.

Normal thyroid cells produce Tg that circulates in the blood, and only thyroid cells produce Tg in the body. In some cases, the body produces an antibody against Tg called antiTg. When antiTg is high, the blood Tg test result will be inaccurate, possibly artificially low because a part of Tg has converted to antiTg.