Thyroglobulin (Tg)

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Subject: Thyroglobulin (Tg)

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Definition

  • Heterogeneous iodoglycoprotein secreted only by thyroid follicular cells that is involved in iodination and synthesis of thyroid hormones. It is proportional to thyroid mass.

  • Normal value: <55 ng/mL.

Use

  • To assess the presence and possibly the extent of residual or recurrent or metastatic follicular or papillary thyroid carcinoma after therapy. In patients with these carcinomas treated with total thyroidectomy or radioiodine and taking thyroid hormone therapy, Tg is undetectable if functional tumor is absent but is detected by sensitive immunoassay if functional tumor is present. Tg correlates with tumor mass with highest values in patients with metastases to bones and lungs.

  • To diagnose factitious hyperthyroidism: Tg is very low or not detectable in factitious hyperthyroidism and is high in all other types of hyperthyroidism (e.g., thyroiditis, Graves disease).

  • To predict outcome of therapy for hyperthyroidism; higher remission rates in patients with lower Tg values. Failure to become normal after drug-induced remission suggests relapse after drugs are discontinued.

  • To diagnose thyroid agenesis in newborn.

Interpretation

 
TABLE 16–76
Thyroid Function Tests in Various Conditions

Increased In

  • Most patients with differentiated thyroid carcinoma but not with undifferentiated or medullary thyroid carcinomas

  • Hyperthyroidism—rapid decline after surgical treatment; gradual decline after radioactive iodine treatment

  • Silent (painless) thyroiditis

  • Endemic goiter (some patients)

  • Marked liver insufficiency

Decreased In

  • Thyroid agenesis in newborns

  • Total thyroidectomy or destruction by radiation

Limitations

  • A Tg test is not recommended for initial diagnosis of thyroid carcinomas. The presence of Tg in pleural effusions indicates metastatic differentiated thyroid cancer.

  • A Tg test should not be used in patients with preexisting thyroid disorders.

  • Tg autoantibodies: patients' serum must always first be screened for these antibodies (present in <10% of persons). In such cases, Tg mRNA can be measured using RT-PCR.

  • Because Tg autoantibodies can interfere with both competitive immunoassays and immunometric assays for Tg, all patients should be screened for Tg autoantibodies by a sensitive immunoassay; recovery studies are not adequate for ruling out interference by these autoantibodies.

  • Tg antibodies are present in the majority of patients with Hashimoto thyroiditis but also in approximately 3% of healthy individuals.

  • At least 6 weeks should elapse after thyroidectomy or iodine-125 treatment before a Tg test. Some reports have indicated that Tg levels may remain elevated for several weeks following successful treatment. In this case, serial determinations assessed relative to a posttreatment baseline established for the patient may still be of value in monitoring.

  • Many technical pitfalls in Tg measurement include between-method variability, in appropriate reference ranges, suboptimal functional sensitivity, hook effects, HAMA interferences. RIA method is relatively resistant to TGAB and HAMA influences.

  • A newer HPLC-MS method is offered by many commercial labs and can be used in suspicious TGAB interference cases.

 
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