Similar to T4, T3 is also highly-bound to proteins (99.7%) in serum. Serum T3 levels are often 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 from a lack of conversion from T4 to T3 due to reduced 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 can determine the unbound quantities of T3 down to picogram levels. As such, the determination of FT3 levels is considered useful in the assessment of thyroid disease differentiation.
A suggested FT3 reference interval is shown. However, assays from different manufacturers vary widely; reference intervals must be established in each clinical laboratory.
