Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1237 page)

   THBR values can be calculated according to the following equation proposed by the Committee on Nomenclature of the American Thyroid Association.

   See Table 16.77.

   
Normal range:
5.93–13.13 μg/dL.

TABLE 16–77. Free Thyroxine Index in Various Conditions

*Normal even though T
₃
and T
₄
alone are abnormal.

   Use

   This calculated product permits correction of misleading results of T
₃
and T
₄
determinations caused by conditions that alter the thyroxine-binding protein concentration (e.g., pregnancy, estrogens, birth control pills).

   Interpretation

Increased In

   Hyperthyroidism

   States with decreased TBG (e.g., androgen treatment, chronic liver disease), protein loss, or genetically low TBG

Decreased In

   Hypothyroidism

   States with increased TBG (e.g., estrogen treatment, pregnancy, acute hepatitis, genetically high TBG)

   Limitations