Thyroid Function Test (TFT)
Blood tests that check how well your thyroid gland is working.
Quick Summary
To diagnose and monitor underactive or overactive thyroid.
TSH, and often free T4 and T3.
Usually no fasting. If you take levothyroxine, ask whether to take it before the test.
Overview
TSH, made by the pituitary gland, tells the thyroid how much hormone to make. It's the most sensitive first test.
Thyroid tests are commonly done in women, in pregnancy and with tiredness or weight changes.
What It Measures
TSH
High in underactive, low in overactive thyroid.
Free T4
The main thyroid hormone.
Free T3
The active hormone.
How It's Performed
- 01
Preparation
Usually no special preparation. Tell the staff about medicines you take.
- 02
Blood sample
A small amount of blood is taken from a vein in your arm with a thin needle.
- 03
Afterwards
Press on the spot for a minute. A small bruise is common; you can go back to normal activities.
Reference Ranges
Note: Normal ranges vary by laboratory. Consult your report for the specific reference used.
| Parameter | Typical Range (Adult) | Units |
|---|---|---|
| TSH | 0.4–4.0 | mIU/L |
| Free T4 | 0.8–1.8 | ng/dL |
| Free T3 | 2.3–4.2 | pg/mL |
Interpreting Results & Next Steps
Discuss with Doctor
Pregnancy has different targets. Normal ranges differ between laboratories, ages and sexes; always use the range printed on your report and ask your doctor what your results mean.
If values are Low:
Low TSH with high T4/T3 suggests an overactive thyroid.
If values are High:
High TSH with low T4 suggests an underactive thyroid.
Sources and further reading
Last updated 21 October 2026. For more detail, see these trusted health organisations.
- TSH (Thyroid-stimulating hormone) Test(opens in a new tab) · MedlinePlus, US National Library of Medicine
- Thyroxine (T4) test(opens in a new tab) · MedlinePlus, US National Library of Medicine