TFT

Alternative names, keywords

Thyroid function tests, thyrotropin, thyroxine, tri-iodothyronine, free T₄, free T₃, fT4, fT3.

Samples required

● Clotted blood (gold cap, 5 mL tube). Smaller tubes are available for paediatric samples.

Test indications

Investigation and monitoring of thyroid dysfunction.

Click here for NICE guideline NG145 Thyroid disease: assessment and management.

Test information

Routine TFT requests will get TSH as a first line screen. If the TSH is within the age specific reference interval only the TSH result will be analysed and reported.

Free T4 will be added automatically if the TSH result is outside the reference interval and for all children (<18 years).

Free T4 should be requested separately in the following situations:

Pregnancy
Patients with known pituitary disease or suspected pituitary dysfunction
Amiodarone treatment
Treatment of thyrotoxicosis or monitoring of other acute thyroid presentations

Samples can referred for analysis by alternative methods if interference is suspected; either to Biochemistry at Worthing for a quick check on a different method or to Addenbrooke's* for more detailed interference studies. Please contact the Duty Biochemist for further information.

A copy of the form for the Addenbrooke's service can be downloaded here. _{(LFc-BSA-TFTreferral, revision 1, last reviewed 18.7.2023)}

Referred to TFT Referral Service, Dept. of Clinical Biochemistry, Addenbrooke's Hospital, Cambridge CB2 0QQ

Factors affecting test performance/ results

All tests

Samples should not be taken from patients receiving therapy with high biotin doses (i.e. > 5 mg/day) until at least 8 hours following the last biotin administration.

In rare cases, interference due to extremely high titres of antibodies to analyte-specific antibodies, streptavidin or ruthenium can occur.

TSH

The presence of autoantibodies may induce high molecular weight complexes (macro-TSH) which may cause unexpected high values of TSH.

Free T₄ and free T₃

Any influence that might affect the binding behaviour of the binding proteins can alter the result of the fT₄ test (e.g. drugs, non-thyroidal illness) or patients with from FDH (Familial Dysalbuminemic Hyperthyroxinaemia).

In in vitro studies furosemide caused elevated fT₄ and fT₃ at the daily therapeutic dosage level.

Free T₄
The test cannot be used in patients receiving treatment with lipid-lowering agents containing D-T4. If the thyroid function is to be checked in such patients, the therapy should first be discontinued for 4-6 weeks to allow the physiological state to become re-established.

Autoantibodies to thyroid hormones can interfere with the assay.

Results should always be assessed in conjunction with the patient’s medical history, clinical examination and other findings.

Reference range

Adults

TSH	0.27 - 4.20 mU/L	_{Source: Roche Cobas® TSH kit insert 05-2015, V22.0}
Free T₄	11.1 - 22 pmol/L	_{Roche Cobas® FT4 kit insert 05-2015, V2.0 (upper limit), in-house data (lower limit)}
Free T₃	3.1 - 6.8 pmol/L	_{Source: Roche Cobas® FT3 kit insert 03-2015, V2.0}

Paediatric

	0 to 6 days	6 days to ≤ 3 months	3 months to ≤ 12 months	1 to ≤ 6 years	6 to ≤ 11 years	11 to ≤ 20 years
TSH (mU/L)	0.70 - 15.2	0.72 - 11.0	0.73 - 8.35	0.70 - 5.97	0.60 - 4.84	0.51 - 4.30
Free T₄ (pmol/L)	11.0 - 32.1	11.5 - 28.4	11.9 - 25.7	12.3 - 22.8	12.5 - 21.5	11.6* - 21.0
Free T₃ (pmol/L)	2.7 - 9.7	3.0 - 9.3	3.3 - 9.0	3.7 - 8.5	3.9 - 8.0	3.9 - 7.7

_{Source: Roche Reference Intervals for Children and Adults Elecsys Thyroid Tests, 2009, 04640292001.}_{* lower limit from local data}

Pregnancy

	1st trimester	2nd trimester	3rd trimester
TSH (mU/L)	0.3 - 4.6	0.4 - 4.1	0.2 - 3.2
Free T₄ (pmol/L)	12.1 - 19.6	9.6 - 17.0	8.4 - 15.6
Free T₃ (pmol/L)	3.8 - 6.0	3.2 - 5.5	3.1 - 5.0

_{Source: Roche Reference Intervals for Children and Adults Elecsys Thyroid Tests, 2009, 04640292001}

Turnaround time

Same day (routine tests)

Enquiries

Biochemistry (Automation)

Test A-Z Index.