Similar to T4, T3 is also highly-bound to proteins (99.7%) in serum. Serum T3 levels are often found to be within normal limits in many cases of hypothyroidism, due to the hyperstimulation of the functioning thyroid tissue with TSH. For this reason, T3 is not considered to be a sensitive indicator of hypothyroidism. Total T3 levels are also of little clinical value in severely ill hospitalized patients, as values may be decreased due to lack of conversion from T4 to T3 due to decreased metabolic needs. Total T3 analysis has been measured by several chemical and immunochemical methods, but they have been too insensitive to detect subtle changes and insufficient levels to assist in the clinical diagnosis of thyroid disease.
FT3 assays are highly sensitive and able to determine the unbound quantities of T3 down to picogram levels. As such, determination of FT3 levels are considered useful in the assessment of thyroid disease differentiation.
A suggested FT3 reference interval is shown, however assays from different manufacturers are known to vary widely; reference intervals must be established in each clinical laboratory.