Thyroglobulin (Tg) and Tg Antibody (TgAb) Testing: Purpose, Procedure and Understanding the Levels
The measurement of the protein Thyroglobulin in blood abbreviated as Tg, is an important laboratory test for checking whether a patient still has some thyroid present after a thyroid surgery as a treatment of thyroid cancer.
Tg can only be made by the thyroid gland, either produced from remaining normal part or the tumorous part. Therefore, when a patient has had their thyroid completely removed, the measurement of Tg in a blood sample can be used to check whether there is any tumor left behind.
Thyroglobulin testing is primarily used to evaluate the effectiveness of treatment for thyroid cancer and to monitor for recurrence. It is not helpful for detecting thyroid cancer. However, it is an indicator of disease status.Thyroglobulin levels are found to decrease with successful treatment. But they rise again upon recurrence. The test is also used in the workup of hyperthyroidism and hypothyroidism. In children with normal thyroid function, thyroglobulin may be used to estimate both deficient and excess thyroid intake.
Thyroglobulin is measured from the serum after drawing a blood sample. The following things should be taken care before the test:
Although it is usually ordered as a single test, thyroglobulin may also be part of the complete thyroid panel, which consists of of thyroid hormone levels and antibody.
Serum thyroglobulin is affected by iodide availability and intake. Therefore, thyroglobulin reference ranges are determined according to geographical indices of iodide levels.
Thyroglobulin is produced by the normal thyroid gland, as well as the thyroid gland affected by Graves disease, thyroiditis, and differentiated thyroid carcinoma. thyroglobulin levels are increased in the circulation, usually above 100 ng/mL in persons with cancer.
Tg levels will be high in persons with a goiter, a history of cigarette smoking, personal or family history of thyroid disease, the presence of thyroid autoantibodies (TgAb) and or thyroid peroxidase antibodies(TPOAb), and a serum TSH level of less than 0.5 mIU/L or greater than 2 mIU/L.
The absence of Tg in a blood sample is usually an indication of no left behind tumor for a patient who has had thyroid surgery to remove their thyroid gland containing a cancerous growth. However, many patients still have measurable levels of Tg in their blood after surgery which could be from a small amount of normal thyroid left behind. Therefore, a measurable level of Tg does not necessarily indicate the presence of tumor. To make Tg measurements a better marker for any tumor left behind, a small dose of radioiodine is given after the surgery to diminish the last remaining part of the normal thyroid gland.
Thyroid Stimulating Hormone or TSH is released by the pituitary which in turn drives the thyroid gland to produce thyroid hormones and as a by-product, release Tg into the blood. TSH is responsible for the growth of most thyroid tumors.
When TSH is high, Tg levels will also be high. Thyroxine medicine, such as, synthroid, levoxyl, unithroid can be taken to keep TSH levels low.
A high Tg level before surgery does not necessarily indicate that a tumor is present. However, when a biopsy suggests that the growth is cancerous, and Tg level is high before surgery, it means that the tumor makes Tg. In this case Tg can be used as a sensitive tumor marker test after surgery. When Tg is detected post-operatively in such patients despite removal of all normal thyroid, this could indicate that a large amount of tumor is still present.
The cancerous growth might be unable to efficiently make Tg, when a patient has a low Tg value before surgery. In this case Tg can not be used as a sensitive tumor marker test after surgery.
Rather than than any one Tg result, measurement of Tg level over six months or yearly intervals is important. Blood samples are usually taken for Tg measurement while the patient is on their daily dose of thyroxine medication after surgery.
The factors that interfere with the measurement of thyroglobulin levels include:
Some of thyroid cancer patients have antibodies to Tg that circulate in their blood. These antibodies are abbreviated as TgAb which interferes with the measurement of Tg by most methods.
The incorrectly high or low values due to TgAb depends on the type of Tg method used by the laboratory. Without measurement of thyroid antibodies, the values obtained for thyroglobulin may prove misleading. Thyroid antibodies that have to be measured in certain types of thyroid testing include the thyroid peroxidase antibody (TPOAb), thyroglobulin antibody (TgAb), and thyroid-stimulating hormone receptor antibody (TRAb). In case of differentiated thyroid cancer, only the thyroglobulin antibody needs to be measured.