Thyroid Hormone–Binding Ratio (THBR)

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 " пїЅ13.13 пїЅ пїЅg/dL.

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TABLE 16 " пїЅ77Free Thyroxine Index in Various ConditionsView LargeTABLE 16 " пїЅ77Free Thyroxine Index in Various Conditions Condition T3 T4 Free Thyroxine Index (T7) (T3 Uptake пїЅ пїЅ T4) Normal Range 24 " пїЅ36 4 " пїЅ11 96 " пїЅ396 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)