T4 (thyroxine) is converted to T3 in peripheral tissues; approximately 20% is synthesized by follicular cells. Most T3 is transported bound to protein; only 0.3% is in free unbound state (see Table 16.76 and Fig. 16.5).
Normal range:
Total T3: 87 " 178 ng/dL
Free T3: 2.5 " 3.9 pg/mL
Use
Diagnosis of T3 thyrotoxicosis (when TSH is suppressed but T4 is normal) or cases in which FT4 is normal in the presence of symptoms of hyperthyroidism.
Evaluating cases in which FT4 is borderline elevated.
Evaluating cases in which overlooking diagnosis of hyperthyroidism is undesirable (e.g., unexplained atrial fibrillation).
Monitoring the course of hyperthyroidism.
Monitoring T4 replacement therapy " is better than T4 or FT4, 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 T3 gives corrected values in patients in whom the total T3 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 T3 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 FT4 remains normal
Limitations
Serum T3 parallels FT4; is early indicator of hyperthyroidism but TSH is better.
Not recommended for diagnosis of hypothyroidism; decreased values have minimal clinical significance.
More than 99% of the total concentration of T3 and T4 is bound by serum proteins, which is not available to elicit biologic activity. It is only the free fraction (<1%) that is readily available to bind its receptor and stimulate a response from the target organ or tissues.
Values below the lower limit of the expected value range can be caused by a number of conditions, including nonthyroidal illness, acute and chronic stress, and hypothyroidism.