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
Yes
24 Hr available service
No
Method
Immunoassay
Reference interval
(I) Non-pregnant individual:
| Age | Range |
| 0 - < 6 d | 2.65 - 9.68 pmol/L |
| > 6 d - < 90 d | 3.00 - 9.28 pmol/L |
| > 90 d - < 1 y | 3.30 - 8.95 pmol/L |
| > 1 y - < 6 y | 3.69 - 8.46 pmol/L |
| > 6 y - < 11 y | 3.88 - 8.02 pmol/L |
| > 11 y - 998 y | 3.10 - 6.80 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
Thyroid stimulating hormone (TSH) is the preferred initial test in the assessment of thyroid function.
The primary indication for free T3 is for the diagnosis of T3 toxicosis (that is, suppressed TSH and normal free T4). Although occasionally used as an early or confirmatory test for thyrotoxicosis (free T3 is often elevated before free T4), it is not required to establish this diagnosis.
It is an unreliable test for hypothyroidism, and should not be used.
Result interpretation
Increased levels of free T3 are found in T3 toxicosis and in thyrotoxicosis.
Measurement of uncertainty
Frequency of measurement
Daily