T4 can be converted to either active T3 or inactive reverse T3 (rT3). In significant illness and stress (nonthyroidal illness syndrome), conversion shifts toward rT3, while T3 falls. This pattern can mimic thyroid disease but typically reflects underlying illness rather than primary thyroid dysfunction.
This test has limited clinical utility in most situations and is generally not recommended for routine thyroid evaluation. It may occasionally be ordered in evaluation of unusual thyroid function patterns in critically ill patients or in selected research contexts. Sample type is serum.
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