Skip to page content

We use cookies to provide you with a better service. Carry on browsing if you’re happy with this or read more about our cookie policy and privacy policy.

Dismiss

Thyroid Function Tests (TSH, FT4, FT3)

Clinical Biochemistry

Background Information

  • Thyroid function tests (TFTs) include thyroid stimulating hormone (TSH, also known as thyrotropin), free T4 (FT4; thyroxine) and free T3 (FT3; tri-iodothyronine).
  • These tests are variably indicated for investigation of possible hypo- or hyper- thyroidism, for monitoring of adequacy of thyroid hormone replacement, for monitoring of anti-thyroid treatment, for the investigation of the hypothalamic-pituitary-thyroid axis and for management of thyroid carcinoma.
  • TSH is the frontline investigation of thyroid disease, with FT4 and FT3 being reflexed by the laboratory as appropriate based on the TSH result, previous results, and available clinical details.
  • The Clinical Biochemistry department has a specific testing strategy for Thyroid Function Testing that is based on NICE guidance. This strategy can be found here: GHNHSFT Clinical Biochemistry Thyroid Testing Strategy.pptx
  • If there is a possibility of hypothalamic or pituitary disease, please state this clearly on the request form, as FT4 should be assayed alongside TSH in these cases.
  • TFTs may be misleading in patients with non-thyroidal illness. Unless there is a strong suspicion of thyroid disease, these tests are not advised for hospital in-patients because of the difficulty in result interpretation. TFTs should be delayed until 3 months post acute illness.
  • Please wait at least 2 months after any change in thyroid hormone replacement before repeating blood tests.
  • TSH levels may remain suppressed for many months after treatment for thyrotoxicosis has commenced. FT4 and FT3 provide better information for clinical management.
  • Amiodarone may precipitate changes in TSH, FT4 and FT3. TFTs should be performed before treatment starts and, once stable, every 6 months thereafter.
  • Lithium can interfere with thyroid function and patients on this medication should be regularly monitored, with thyroid function being checked before commencing the drug and then at a minimum of every 6 months thereafter (along with renal function and calcium levels).

Sample Requirements

  • For patients taking high dose biotin therapy (>5mg/day), collect sample at least 8 hours after the last dose.
  • Please note there is a minimum retesting interval of one month for TFTs

For adults, blood taken into a 5mL gold top gel tube (or rust top for the Acute Unit)

5ml gold tube


For children, blood taken into a 3.5mL rust top gel tube

3.5ml rust top tube


For neonates, blood taken into a 0.8mL minicollect lithium heparin tube

Sample tube: pale green cap


Storage/Transport

Send at ambient temperature to the laboratory. If unavoidable, samples can be stored refrigerated overnight.

Required Information

Relevant clinical details including current treatment (especially T4 or T3 replacement, anti-thyroid drugs, amiodarone or lithium), post thyroid surgery, radioiodine, or suspicion of hypothalamic or pituitary disease.

Turnaround Times

The assays are run throughout the day and night.

The turnaround time is normally less than 24 hours.

The test can be ordered as an urgent request.

Reference Ranges

Adult Reference Ranges:

TSH: 0.27 - 4.2 mIU/L

FT4: 12.0 - 22.0 pmol/L

FT3: 3.1 - 6.8 pmol/L

Age-related reference ranges for children apply as displayed in the table below.

Please note there are also pregnancy related reference ranges displayed below.


0 -6 Days 7-14 Days 15 Days - 16 Years
TSH (mIU/L) 0.7 - 15.2 0.72 - 11.0 0.27 - 4.2
Free T4 (pmol/L) 11.0 - 32.0 11.5 - 28.3 12.0 - 22.0
Free T3 (pmol/L) 2.7 - 9.7 3.0 - 9.3 3.1 - 6.8
1st trimester 2nd trimester 3rd trimester
TSH (mIU/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 - 16.0
Free T3 (pmol/L) 3.8 - 6.0 3.2 - 5.5 3.1 - 5.0

Further Information

To learn more about thyroid function testing visit Lab Tests Online

Page last updated: 01/07/2026 | Page last reviewed: 01/07/2026

Back to top