Turnaround times
The quoted turnaround time is from sample receipt in the laboratory, to results authorisation in the Laboratory Information Management system. The times do not include transport of specimen to the laboratory or the administrative process to print and post/email reports. Service users must allow for transport and reporting time when ordering tests.
Clinical background:
Thyroxine (T4) is the principal hormone produced by the thyroid gland and is secreted into the bloodstream under stimulation by thyroid stimulating hormone (TSH). Over 99.5% of the circulating Total T4 is reversibly bound to the major transport proteins thyroxine binding globulin (TBG), thyroxine binding pre-albumin (TBPA) and albumin. In normal patients, a small fraction of the hormone (0.03% of the Total T4) circulates free in the blood and is maintained in equilibrium with T4 bound to these proteins.
T4 acts to increase the metabolic activity of most tissues of the body and is a major precursor to triiodothyronine (T3). The free T4 fraction (FT4) is commonly accepted as the physiologically active material penetrating tissue cells, inducing metabolic stimulation and providing feedback to the hypothalamic-pituitary-thyroid axis. Therefore, free T4 is a direct indicator of thyroid metabolic status. Measurement of free T4 is independent of thyroid-binding protein concentration and thus provides improved correlation with functional thyroid states in the individual patients compared with the measurement of total T4. Free T4 is measured as part of a thyroid function test with TSH, or may be cascaded following an abnormal TSH result.
Specimen container paediatric:
Serum (SST or plain tube)
Specimen container adult:
Serum (SST or plain tube)
Minimum volume paediatric:
0.5 mL blood
Minimum volume adult:
1 mL blood
Sample stability:
Unseparated sample: unknown
Separated sample:
2 days at 20 to 25°C,
8 days at 4 to 8°C,
3 month at -20°C
Transport requirements:
Ambient
Interpretation:
Low TSH with raised FT4 (and/or FT3) is consistent with primary hyperthyroidisim. Raised TSH with low FT4 is consistent with hypothyroidism. ‘Sub-clinical’ or borderline hypo/hyperthyroidism can be associated with a FT4 within the reference range but abnormal TSH. Low FT4 with an inappropriatly low or normal TSH can be associated with secondary hypothyroidism. It is important to note that non-thyroidal illness can cause abnormal thyroid.
Reference ranges:
| Age Range | Free T4 (pmol/L) |
| Adults | 10.0 – 22.0 |
| >11-18 yrs | 12.6 – 21.0 |
| >6-11 yrs | 12.5 – 21.5 |
| >1-6 yrs | 12.3 – 22.8 |
| >3 m-1 yr | 11.9 – 25.6 |
| >6 d-3 m | 11.5 – 28.3 |
| 0-6 d | 11.0 – 32.0 |
Other info:
Lithium heparin plasma samples also acceptable