Free T3, serum
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 t...
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’).
Test Details
-
Discipline:
Biochemistry
Biochemistry
-
Specimen Container Adult:
Serum (SST or plain tube)
Serum (SST or plain tube)
-
Specimen Container Paediatric:
Serum (SST or plain tube)
Serum (SST or plain tube)
-
Minimum Volume Adult:
1 mL blood
1 mL blood
-
Minimum Volume Paediatric:
0.5 mL
0.5 mL
-
Sample Stability:
Unseparated sample: Unknown
Separated sample:
24 hours at 15-25C
2 weeks at 4-8°C
3 months at -20°C
Unseparated sample: Unknown
Separated sample:
24 hours at 15-25C
2 weeks at 4-8°C
3 months at -20°C
-
Transport Requirements:
Ambient
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...
Read MoreLow 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
... Read MoreAge 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
Lithium heparin plasma samples also acceptable
-
Routine Contact Name:
Duty Biochemist
-
Routine Telephone:
Freeman: 0191 244 8889
RVI: 0191 282 9719
Freeman: 0191 244 8889
RVI: 0191 282 9719
-
Routine Email: