Preferred Sample Type
Suitable Specimen Types
- Serum
Sample Processing in Laboratory
Usual
Sample Preparation
Centrifuge
Turnaround Time
2 daysSample Stability
Store at 4 ºC for up to seven days. For longer periods store at -20 ºC.
Free Triodothyronine (FT3)
General Information
Tri-iodothyronine (T3) constitutes approximately 5% of the thyroid hormones in serum. T3 is produced by both the thyroid gland and mono-deiodination of Thyroxine.
(T4) in the peripheral tissues. It is primarily transported by carrier proteins: thyroid binding globulin (TBG), albumin and pre-albumin. T3 has greater metabolic activity than T4, has a shorter half-life and less affinity for TBG. Only about 0.5% of T3 circulates freely in the serum and is considered the biologically active fraction. This is FT3, the measurement of which is is useful in the diagnosis of hyperthyroidism. FT3 measurement has practical importance in thyrotoxic patients (T3 toxicosis) who exhibit elevated FT3 levels while maintaining normal FT4 concentrations.
Free T3 plays an important role in maintenance of the euthyroid state and assists in the diagnosis of thyroid dysfunction. Unlike total T3, free T3 is largely unaffected by variations in thyroid hormone-binding protein levels. Euthyroid patients with elevated concentrations of TBG, as in pregnancy, use of oral contraceptives and estrogen therapy exhibit an increase in total T3 concentrations but not FT3. Euthyroid patients with very low levels of TBG exhibit subnormal levels of total T3 while free T3 concentrations remain essentially “normal”.
Patient Preparation
Patients receiving therapy with biotin (> 5mg/day) should not have samples taken for analysis until at least 8 hours post dose.
Notes
FT3 analysis is only available in certain circumstances.
Samples should be clot-free and free of red cells or other particulate matter. The presence of fibrin may cause erroneous results.
In vitro furosemide at the daily therapeutic dosage level has been shown to depress FT3. Rarely interference occurs via endogenous antibodies directed towards streptavidin ruthenium.
Trimester-specific reference ranges for thyroid function:
|
Abbott reference range |
Trimester 1 |
Trimester 2 |
Trimester 3 |
|
|
TSH (mIU/L) |
0.4 - 4.9 |
0.0878-2.8293 |
0.1998-2.7915 |
0.3070-2.9028 |
|
FT4 (pmol/L) |
9 - 19 |
10.53-18.28 |
9.53-15.68 |
8.63-13.61 |
|
FT3 (pmol/L) |
2.9 – 4.9 |
3.52-6.22 |
3.41-5.78 |
3.33-5.59 |
Reference Range
Please note new reference range from 6/7/20: 2.4 to 6.0 pmol/L
(Source: Abbott Diagnostics)
Specifications
- EQA Scheme?: Yes
-
EQA Status:
NEQAS and WEQAS