Pathology Handbook

Thyroid hormone - free T3

Synonyms

Triiodothyronine

Clinical Indications

Free T3 levels are useful in the differential diagnosis of hyperthyroidism and non-thyroidal illness

TSH levels may be suppressed in both conditions with normal or raised free T4.

Free T3 is not helpful and may be misleading in diagnosis of hypothyroidism.

Free T3 may also be misleading in monitoring T3 replacement because of wide variations that depend on the interval between dose and sampling.
Patients on anti-thyroid treatment may require free T3 levels to identify persistent drug treatment thyrotoxicosis due to isolated T3 excess. 

Part of Profile

Thyroid Function Tests

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Availability

On request.

Free T3 is part of the 'TF3' profile for the investigation and follow-up of hyperthyroid patients.

Free T3 is automatically measured if TSH is suppressed and free T4 is elevated.

Specific Criteria

Not appropriate in hypothyroid patients

Turnaround Time

Daily

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; inappropriate request

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