Definition
- THBR values can be calculated according to the following equation proposed by the Committee on Nomenclature of the American Thyroid Association.
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- See Table 16.77.
- Normal range: 5.93 " �13.13 � �g/dL.
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TABLE 16 " �77Free Thyroxine Index in Various ConditionsView LargeTABLE 16 " �77Free Thyroxine Index in Various Conditions Condition T3 T4 Free Thyroxine Index (T7) (T3 Uptake � � T4) Normal Range 24 " �36 4 " �11 96 " �396 Mean 31 7 217 Hypothyroid 22 3 66 Hyperthyroid 38 12 456 Pregnancy, estrogen use (especially birth control pills) 20 12 240* *Normal even though T3 and T4 alone are abnormal.
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)