Total Thyroxine (TT4) and Total Triiodothyronine (TT3) Methods

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Serum TT4 and TT3 Normal Reference Intervals

Serum TT4 values vary between methods to some extent. Typical reference ranges approximate 58 - 160 nmol/L (4.5-12.6 µg/dL). Likewise, serum TT3 values are method dependent, with reference ranges approximating 1.2 - 2.7 nmol/L (80 -180 ng/dL).

Guideline 10. Serum Total T4 (TT4) and Total T3 (TT3) Measurements

Abnormal serum TT4 and TT3 concentrations are more commonly encountered as a result of binding protein abnormalities and not thyroid dysfunction.

  • Free T4 estimate tests (FT4) are preferred over TT4 measurement when TBG concentration is abnormal. However, FT4 tests but may be diagnostically inaccurate when the affinity of TBG is altered or abnormal T4-binding proteins are present.

  • Total hormone assays (TT4 and TT3) should remain readily available to evaluate discordant free hormone tests.