Thyroid Hormone–Binding Ratio (THBR)

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Subject: Thyroid Hormone–Binding Ratio (THBR)

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Definition

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

 
TABLE 16–77
Free Thyroxine Index in Various Conditions

Use

  • This calculated product permits correction of misleading results of T3 and T4 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

  • Concordance values of T4 and THBR tests suggest altered thyroid function.

  • Discordant variance suggests primary change in TBG in a euthyroid state (e.g., pregnancy)

 
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