Tg Antibodies - Kresser Institute Membership Site

Marker Name: Tg antibodies

REFERENCE RANGES FOR SERUM TG ANTIBODIES:

Laboratory reference range: 0–0.9 IU/mL

DESCRIPTION:

Thyroglobulin (Tg) antibodies are autoantibodies directed against epitopes contained within the thyroglobulin protein. The presence of Tg antibodies in the blood is abnormal. Tg antibodies in the blood often indicate chronic autoimmune thyroiditis.¹

Thyroglobulin is a glycoprotein composed of two identical subunits that are noncovalently bound together.² Thyroglobulin is integral to thyroid hormone formation, which takes place within the lumen of thyroid follicles. It is the principal storage form of thyroid hormones within the thyroid gland. Approximately 25 mg of thyroglobulin is hydrolyzed each day to yield 100 µg of thyroxine (T4).³ A relatively minute quantity, 100 µg, of thyroglobulin is released from the thyroid daily.² Thyroglobulin is the major protein in the thyroid gland, by weight, and is the principal component of colloid, where thyroid hormone is stored within the thyroid gland.²

The mechanism by which the immune system produces autoantibodies against Tg is unknown, but leading hypotheses believe that an initial insult initiates the autoimmune process. A viral illness is considered the most common cause, but injury to the thyroid gland, pregnancy, and excessive iodine intake may also trigger this process.^1,4-7 B cells and T cells respond inappropriately to a similar epitope on a virus particle or recognize “self” HLA class II molecules as foreign when presented to them by thyroid follicle cells.^1,8,9

Tg antibodies are usually detected in the same clinical situations as TPO antibodies. Tg and TPO antibodies are present in roughly 90 percent of people with Hashimoto’s thyroiditis and 50 to 80 percent of people with Graves’ disease.¹⁰ Tg antibodies, like TPO antibodies, may be present in subclinical hypothyroidism, indicating an elevated risk of progressing to overt hypothyroidism.¹¹ Laboratory tests used to detect Tg antibodies are less sensitive and only as specific as those used to detect thyroid peroxidase (TPO) antibodies. Therefore, Tg antibody tests are often considered a secondary test for autoimmune hypothyroidism after TPO antibodies.^12,13

Thyroglobulin levels (not Tg antibodies) are used to monitor patients treated for differentiated thyroid carcinoma.^14,15 However, if patients have autoantibodies directed against the thyroglobulin molecule (i.e., Tg antibodies), the use of thyroglobulin levels as a tumor biomarker may be compromised.¹³ Tg antibodies are in fact elevated in 20 percent of patients with thyroid cancer.^16,17Thus, in patients with treated thyroid cancer, both thyroglobulin and Tg antibodies are measured together. Changes in Tg antibodies may be a more rapid indicator of recurrent disease than changes in thyroglobulin levels.

Women with pregnancy-related thyroiditis or postpartum thyroiditis who also have elevated Tg antibodies are at greater risk of developing permanent autoimmune hypothyroidism.¹³ As with TPO antibodies, Tg antibodies can be passed across the placenta to fetuses, which may transiently disrupt thyroid function (i.e., cause hypothyroidism) during the neonatal period.^18,19

The Tg antibodies test may be ordered in conjunction with TPO antibodies and other thyroid markers (e.g., serum TSH, total T3, and free T4 levels).²⁰

PATHOLOGICAL/CONVENTIONAL RANGE INDICATIONS:

High in:^1,17,21-23

Hashimoto’s thyroiditis/chronic autoimmune hypothyroidism
- Goitrous autoimmune thyroiditis
- Atrophic autoimmune thyroiditis
Graves’ disease
Differentiated thyroid carcinoma
Subclinical hypothyroidism
Pregnancy (abnormal)
Neonatal period (transient)
Postpartum thyroiditis
Painless (silent) thyroiditis

Low in:

Not applicable

FUNCTIONAL RANGE INDICATIONS:

High in:

No functional range

Low in:

No functional range

References: