Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1234 page)

0, absent; A, abnormal; D, decreased; I, increased; N, normal; NA, not useful; TT
₄
, 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.