Pathology Handbook

Thyroid hormone - free T4

Synonyms

Thyroxine

Clinical Indications

Free T4 levels are required for definitive assessment of thyroid status when TSH is abnormal and if pituitary dysfunction is supsected.

During long-term replacement or suppressive therapy with thyroxine, TSH is the best index of appropriate dose (unless treatment of hypothyroidism due to pituitary or hypothalamic disease).

Free T4 levels are also required in early treatment of hyperthyroid disease, since TSH levels may remain suppressed for several months after normalisation of free T4.

Part of Profile

Thyroid Function Tests

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Request on ICE

Availability

On request.

Free T4 is automatically measured if TSH level is abnormal or in suspected pituitary dysfunction.

Specific Criteria

None

Turnaround Time

Same Day (Monday to Friday)

Specimen

Serum in yellow top (SST) tube

Volume

0.5 ml

Container

Vacutainer gold top

Collection

No specific collection conditions.

Causes for Rejection

Unlabelled or inadequately labelled sample

Repeat request within 7 days

Reference Range

Reference ranges only apply without non-thyroidal illness.

TSH levels may be suppressed or slightly raised in patients with acute non-thyroidal illness and tests should be repeated when the patient is not acutely ill.

Age TSH (mU/L) FT4 (pmol/L) FT3 (pmol/L)
Newborn 0.70 - 15.2 11.0 - 32.0 2.7 - 9.7
6 days to 3 months 0.72 - 11.0 11.5 - 28.3 3.0 - 9.3
4 to 12 months 0.73 - 8.35 11.9 - 25.6 3.3 - 8.9
1 to 6 years 0.70 - 6.00 12.3 - 22.8 3.7 - 8.5
7 to 11 years 0.60 - 4.85 12.5 - 21.5 3.9 - 8.0
12 to 20 years 0.50 - 4.30 12.6 - 21.0 3.9 - 7.7
Adults 0.27 - 4.20 12.0 - 22.0 3.1 - 6.8