Pathology tests database

Sample and container requirements

Serum Lithium Heparin Plasma (paediatrics only)

Minimum volume required

Special requirements

None

Patient preparation

Samples from patients receiving high biotin doses (i.e. >5 mg/day) should be taken at least 8 hours post-dose. Patients receiving treatment with lipid lowering agents containing D-T4 should have the treatment suspended 4 - 6 weeks prior to fT4 measurements.

Sample viability

Turnaround time

This assay is run throughout the day and night. The in-lab turnaround time is normally less than 24 hours.

Reference range

9 – 19 pmol/L (Source: Abbott Diagnostics) In pregnancy: Trimester 1: 10.53 – 18.28 pmol/L Trimester 2: 9.53 – 15.68 pmol/L Trimester 3: 8.63 – 13.61 pmol/L (Source: Stricker et al., 2007, doi: 10.1530/EJE-07-0249)

Clinical guidance

Free T4 is measured with TSH when thyroid function disorders are suspected. The determination of fT4 is also suitable for monitoring thyrosuppressive therapy. Please refer to LabTestsOnlineUK to learn more about fT4. https://labtestsonline.org.uk/tests/ft4

Notes

fT4 is usually performed only after laboratory reflex consideration of first line TSH analysis. Auto-antibodies to thyroid hormones can interfere with the assay.