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:
Triiodothyronine (3,5,3-L-triiodothyronine, T3) is a hormone synthesised and secreted from the thyroid gland, and formed by peripheral de-iodination of thyroxine (T4). T3 and T4 are secreted into the circulation in response to thyroid stimulating hormone (TSH) and play an important role in regulating metabolism. The T3 and T4 secretion are regulated by a negative feedback mechanism involving the thyroid gland, pituitary gland and hypothalamus.
In the circulation, 99.7% of T3 is reversibly bound to transport proteins, primarily thyroxine-binding globulin (TBG) and to a lesser extent albumin and pre-albumin. The remaining T3 does not bind to transport proteins, but is free in the circulation. This unbound fraction of the total T3 concentration is free triiodothyronine (free T3, FT3). Unbound T3 is metabolically active. Measuring the free hormone has the advantage of being independent of changes in the concentrations and binding properties of the serum binding proteins. The principal indication for measurement of FT3 is in the diagnosis of hyperthyroidism as in some cases, FT3 can become elevated while FT4 remains in the reference range (‘T3 toxicosis’).
Specimen container paediatric:
Serum (SST or plain tube)
Specimen container adult:
Serum (SST or plain tube)
Minimum volume paediatric:
0.5 mL
Minimum volume adult:
1 mL blood
Sample stability:
Unseparated sample:
Unknown
Separated sample:
24 hours at 15 to 25C
2 weeks at 4 to 8°C
3 months at -20°C
Transport requirements:
Ambient
Interpretation:
Low TSH with raised FT3 (and/or FT4) is consistent with primary hyperthyroidisim. It is important to note that non-thyroidal illness can cause abnormal thyroid function test results, which can make interpretation difficult in hospitalised patients.
Reference ranges:
| Age Range | Free T3 (pmol/L) |
| Adults | 3.1 – 6.8 |
| >11-18 yrs | 3.9 – 7.7 |
| >6-11 yrs | 3.9 – 8.0 |
| >1-6 yrs | 3.7 – 8.5 |
| >3 m-1 yr | 3.3 – 9.0 |
| >6 d-3 m | 3.0 – 9.3 |
| 0-6 d | 2.7 – 9.7 |
Other info:
Lithium heparin plasma samples also acceptable