Although produced in smaller quantities than T₄, triiodothyronine (T₃) is physiologically more significant. The competitive protein-binding techniques that are useful in measuring T₄ are not used to measure T₃ because it is present in smaller amounts and has less affinity for TBG than for T₄. Thus, T₃ is measured only by radioimmunoassay (T₃ RIA).

As with T₄, most T₃ (99.7 percent) in the serum is bound to TBG. The remainder circulates as unbound ("free") T₃ (FT₃) and is responsible for all of the physiological activity of T₃. Because FT₃ is not dependent on normal levels of TBG, as is the case with total T₃, FT₃ levels are the most accurate indicators of thyrometabolic activity. FT₃ levels may be calculated by multiplying total T₃ levels by the T₃ uptake ratio.

• Support for diagnosing hyperthyroidism in clients with normal T₄ levels, with early hyperthyroidism and T₃ thyrotoxicosis indicated by elevated T₃ levels in the presence of normal T₄ levels
• Support for diagnosing "euthyroid sick" syndrome in severely ill clients with protein deficiency, as indicated by low T₃ levels, normal FT₃ levels, and elevated rT₃ levels⁵⁰

Nursing Care Before the Procedure

Client preparation is essentially the same as that for any study involving collection of a peripheral blood sample (see Appendix I).

A venipuncture is performed and the sample collected in a red-topped tube. The sample should be handled gently to avoid hemolysis and transported promptly to the laboratory.

Nursing Care After the Procedure

Care and assessment after the procedure are the same as for any study involving the collection of a peripheral blood sample.