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free thyroxine index (fT4I)
Reference interval:
- 0.8-2.4 ng/dL
- 10-31 pmol/L
Increases:
- hyperthyroidism
Decreases:
- hypothyroidism
Specimen:
- serum
- refrigerate immediately
- stable for 7 days at room temp, but storage at 4 C preferred
Method:
T4 Immunoassay / THBR.
The thyroid hormone binding ratio (THBR) has replaced T3 uptake in most laboratories because of internal standardization & ease of interpretation. In the isotopic T3-uptake, resin (or other solid phase binding material) is added to the patient's serum, & the proportion of label bound by the solid phase matrix (the uptake) is measured. The result for the patient's serum is divided by that for a reference serum to give a ratio. The ratio is proportional to the free fraction of T3 or T4 in serum.
% uptake by patient's serum
THBR = ----------------------------
% uptake by reference serum
Age Reference interval (unitless)
20-49 0.85-1.14
50-90 0.78-1.15
The free thyroxine index provides an estimate of the of the biologically active (free) T4. It is calculated by multiplying the total serum thyroxine (T4) by the THBR.
fT4I = T4 (total) x THBR
Related
thyroxine (T4)
thyroxine [T4] (free) in serum
General
general chemistry test
References
- Clinical Guide to Laboratory Tests, 3rd edition, NW Tietz
ed, WB Saunders, Philadelphia, 1995
- Panel of 3 tests
Laboratory Test Directory ARUP: 70141