The FTI is a mathematical calculation used to correct the estimated total T4 for the amount of TBG present. The FTI is useful in the diagnosis of hyperthyroidism and hypothyroidism, especially in patients with known or suspected abnormalities in T4-binding protein levels. In such cases, blood levels and clinical signs may seem contradictory unless both T4 and TBG are considered as interrelated parameters of thyroid status. Measurement of FT4 also gives a more accurate picture of the thyroid status when the TBG is abnormal in pregnant women or persons being treated with estrogen, androgens, phenytoin, or salicylates.
Formula for FTI: FTI = T4 divided by thyroid binding capacity
The FTI permits meaningful interpretation by balancing out most nonthyroidal factors. In recent years, this parameter has lost popularity and is of dubious value.
Calculation helps distinguish the patient as being in a euthyroid, hyperthyroid, or hypothyroid state. Direct measurement has replaced using this calculation in most laboratories.
Pretest Patient Care
Inform the patient about the test purpose and method of calculation.
Follow Chapter 1 guidelines for safe, effective, informed pretest care.
Posttest Patient Care
Be prepared to counsel patient if treatment is required. Review test results; report and record findings. Modify the nursing care plan as needed.
Follow Chapter 1 guidelines for safe, effective, informed posttest care.
Levels fluctuate in pregnancy.
see Appendix E for drugs that affect test outcomes.