free thyroxine index (fT4I)
Reference interval
Increases
Decreases
Methods
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
| 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
Specimen
- serum
- refrigerate immediately
- stable for 7 days at room temp, but storage at 4 C preferred
More general terms
Additional terms
References
- ↑ Clinical Guide to Laboratory Tests, 3rd edition, NW Tietz ed, WB Saunders, Philadelphia, 1995
- ↑ Panel of 3 tests Laboratory Test Directory ARUP: http://www.aruplab.com/guides/ug/tests/0070141.jsp