para>0, absent; A, abnormal; D, decreased; I, increased; N, normal; NA, not useful; TT4, total thyroxine; V, variable; VD, variable decrease; VI, variable increase; X, contraindicated. Underlined test indicates most useful diagnostic change. *Forms of nonthyroidal illness (euthyroid sick syndrome).
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.