Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1260 page)

   Evaluating cases in which overlooking diagnosis of hyperthyroidism is undesirable (e.g., unexplained atrial fibrillation).

   Monitoring the course of hyperthyroidism.

   Monitoring T
₄
replacement therapy—is better than T
₄
or FT
₄
, but TSH is preferred to both.

   Predicting outcome of antithyroid drug therapy in patients with Graves disease.

   Evaluation of amiodarone-induced thyrotoxicosis.

   Good biochemical indicator of severity of thyrotoxicity in hyperthyroidism.

   Free T
₃
gives corrected values in patients in whom the total T
₃
is altered on account of changes in serum proteins or in binding sites (e.g., pregnancy), drugs (e.g., androgens, estrogens, birth control pills, phenytoin [Dilantin]), altered levels of serum proteins (e.g., nephrosis).

   Interpretation

Increased In

   Elevated concentrations of T
₃
occur in Graves disease and most other classical causes of hyperthyroidism.

Decreased In

   Decreased concentrations occur in primary hypothyroid diseases such as Hashimoto thyroiditis and neonatal hypothyroidism or secondary hypothyroidism due to defects at the hypothalamohypophyseal level

   May decrease by ≤25% in healthy older persons while FT
₄
remains normal

   Limitations

   Serum T
₃
parallels FT
₄
; is early indicator of hyperthyroidism but TSH is better.