Adult patient:
Clotted blood tube
(Serum gel separator clot activator tube)
Minimum blood volume: 5 mL
Paediatric patient:
Paediatric clotted blood tube
Number of vials: 2
Minimum blood volume: 1.6 mL
Authorisation code required
No
24 Hr available service
No
Method
Immunoassay
Reference interval
(I) Non-pregnant individual:
| Age | Range |
| 0 - < 6 d | 11.0 - 32.0 pmol/L |
| > 6 d - < 90 d | 11.5 - 28.3 pmol/L |
| > 90 d - < 1 y | 11.9 - 25.6 pmol/L |
| > 1 y - < 6 y | 12.3 - 22.8 pmol/L |
| > 6 y - < 11y | 12.5 - 21.5 pmol/L |
| > 11 y - 998 y | 12.0 - 22.0 pmol/L |
(Source:
Reference Intervals for Children and Adults 2009: Elecsys Thyroid Tests)
(II) During pregnancy (gestational age-specific):
Gestation (week) |
sTSH (mIU/L) |
FT3 (pmol/L) |
FT4 (pmol/L) |
< 5 |
0.38 - 4.84 |
3.49 - 5.70 |
11.6 - 23.0 |
> 5 - 6 |
0.22 - 4.11 |
3.54 - 5.79 |
11.9 - 23.8 |
> 6 - 7 |
0.09 - 3.35 |
3.58 - 5.90 |
12.1 - 24.5 |
> 7 - 8 |
0.05 - 3.02 |
3.59 - 5.92 |
12.1 - 24.5 |
> 8 - 9 |
0.03 - 2.88 |
3.59 - 5.92 |
12.1 - 24.5 |
> 9 - 10 |
0.03 - 2.88 |
3.57 - 5.87 |
11.9 - 23.9 |
> 10 - 11 |
0.04 - 2.93 |
3.54 - 5.81 |
11.7 - 23.3 |
> 11 - 12 |
0.05 - 3.03 |
3.51 - 5.74 |
11.4 - 22.8 |
> 12 - 13 |
0.06 - 3.14 |
3.47 - 5.66 |
11.2 - 22.2 |
> 13 - 14 |
0.08 - 3.27 |
3.43 - 5.57 |
11.0 - 21.6 |
> 14 - 15 |
0.09 - 3.40 |
3.39 - 5.49 |
10.7 – 21.0 |
> 15 - 16 |
0.11 - 3.52 |
3.35 - 5.40 |
10.5 - 20.5 |
> 16 - 20 |
0.15 - 3.78 |
3.25 - 5.20 |
10.1 - 19.4 |
> 20 - 32 |
0.23 - 4.18 |
3.03 - 4.75 |
9.2 - 17.4 |
> 32 - 36 |
0.31 - 4.54 |
2.94 - 4.56 |
9.0 - 17.0 |
> 36 |
0.44 - 5.07 |
2.97 - 4.62 |
9.2 - 17.4 |
(Source: Thyroid. 2020 Apr;30(4): 598 - 608)
Clinical indication
Investigation of thyroid function, as an adjunct to thyroid stimulating hormone (TSH) , which is the preferred test for assessment of thyroid status.
Monitoring patients on thyroid replacement therapy
Result interpretation
The results should always be interpreted in association with TSH.
Free T4 is elevated in hyperthyroidism. Levels are usually depressed in hypothyroidism, but TSH elevation is a more sensitive indicator.
A patient may be hypothyroid, with TSH levels greater than twice the reference interval but free T4 within the reference interval. Primary hypothyroidism cannot be excluded by free thyroxine alone.
Free T4 may be reduced or increased in non-thyroidal illness (sick euthyroid syndrome), but TSH levels are not abnormal.
Total T4 is occasionally measured (as part of the free thyroxine index) when free T4 results are suspected, on clinical grounds, of being anomalous.
Measurement of uncertainty
Frequency of measurement
Daily