FT4 (free T4)
Code:
FT4
Sample Type:
Serum (Gold Top)
Minimum volume: 1 mL
Ref Ranges/Units:
Adult: 10 – 22 pmol/L
Results <5 pmol/L or >50 pmol/L will be phoned to the requesting clinician.
Turnaround:
Same Day (60 minutes from receipt for Urgent Samples)
Stability:
5 days at 20‑25°C
7 days at 2‑8°C
30 days at -20°C
Freeze only once
Special Precautions/Comments:
Please indicate on request form if patient is on any thyroid treatment.
Additional Information:
T4 (thyroxine) is the major thyroid hormone secreted in response to thyroid stimulating hormone (TSH). T4 is also a precursor molecule to the thyroid hormone T3 (triiodothyronine). The majority (>99%) of T4 circulates bound to transporter proteins such as thyroxine binding globulin (TBG) and albumin. The remaining T4 is free in the blood and biologically available to carry out its metabolic function.
Free T4 is therefore measured to assess thyroid function in the investigation of hypothyroidism and hyperthyroidism. Free T4 is added to a thyroid function test if the TSH is outside the reference range.
An increased TSH with low free T4 is indicative of thyroid insufficiency (primary hypothyroidism). A low free T4 with a low TSH is suggestive of secondary or tertiary hypothyroidism. In contrast, increased free T4 with a suppressed TSH is indicative of thyroid overactivity (hyperthyroidism). A normal free T4 in the presence of an abnormal TSH is suggestive of subclinical (‘borderline’) thyroid dysfunction. Non-thyroidal illness can cause transiently abnormal thyroid function tests.
South of Tyne & Wear Pathology Centre
