Newcastle Laboratories

Free T4, serum

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 Tota...

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.

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Test Details

  • Discipline:

    Biochemistry

    Biochemistry

  • Specimen Container Adult:

    Serum - SST

    Serum - SST

  • Specimen Container Paediatric:

    Serum-Plain tube

    Serum-Plain tube

  • Minimum Volume Adult:

    1 mL blood

    1 mL blood

  • Minimum Volume Paediatric:

    0.5 mL blood

    0.5 mL blood

  • Sample Stability:

    Unseparated sample: unknown

    Separated sample: 2 days at 20-25C, 8 days at 4-8°C, 3 month at -20°C

    Unseparated sample: unknown

    Separated sample: 2 days at 20-25C, 8 days at 4-8°C, 3 month at -20°C

  • Transport Requirements:

    Ambient

    Ambient

  • Interpretation:

    Refer to Thyroid regional assessment and management plan

    Low TSH with raised FT4 (and/or FT3) is consistent with primary hyperthyroidisim. Raised TSH with low FT4 is consistent with hypothyroidism....

    Refer to Thyroid regional assessment and management plan

    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.

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  • 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

    ...

    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

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  • Factors Affecting Result:

    Heterophilic antibodies can interfere with immunoassays.

    Heterophilic antibodies can interfere with immunoassays.

  • Other Info:

    Plasma- Lithium Heparin also acceptable

    Plasma- Lithium Heparin 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:

Availability:

Available 24/7

Sites of analysis: FH and RVI

Turn Around:

Urgent: within 3h

Non-urgent: 2 full access days

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