Free T4 (free thyroxine) blood test
Quick answer: A free T4 test measures the unbound form of thyroxine, the main hormone made by the thyroid gland. Doctors order it with TSH to check for an underactive or overactive thyroid and to adjust thyroid treatment.
- SampleBlood
- TakesAbout 5 minutes
- Before the testNo special preparation is usually needed; if you take thyroid tablets, ask your doctor whether to give the sample before your dose.
- In fertility careThyroid problems can affect ovulation and pregnancy, so thyroid levels are commonly checked before fertility treatment and in pregnancy.
How the test is done
- 1If you take thyroid tablets, ask whether to give the sample before your dose
- 2The inside of your elbow is cleaned
- 3A needle draws blood into a tube
- 4The lab measures free T4, usually with TSH
What your results can mean
Reading your report
Ranges differ between labs and in pregnancy, so discuss your result with your doctor.
If the result is outside the usual range
A high level can be seen with an overactive thyroid or too high a dose of thyroid tablets. A low level can be seen with an underactive thyroid, such as Hashimoto's thyroiditis, or a pituitary problem.
Who usually has this test
People with symptoms of thyroid problems, women planning pregnancy or IVF, pregnant women, and people on thyroid treatment.
Risks and aftercare
You can take your usual thyroid tablet after the sample if your doctor advised waiting.
Names and options
Also called: FT4, free T4, free thyroxine
Listed by the hospital as
- FREE T4 (FREE THYROXINE), SERUM
Questions about this test
What is the difference between free T4 and TSH?
TSH is the signal from the pituitary gland, while free T4 is the hormone the thyroid makes in response. Doctors usually read them together to tell where a thyroid problem lies.
Why are thyroid levels checked before IVF?
Thyroid problems can affect ovulation and raise the chance of miscarriage. Correcting them before treatment supports a healthy pregnancy.
Does pregnancy change free T4 results?
Yes. Thyroid hormone levels change in pregnancy, and labs may use pregnancy-specific ranges. Your doctor will read your result with this in mind.
Related tests
Tamara Hospital & IVF Centre, Rajajinagar
34/3, 10th Cross Road, opp. Vardhaka Sardar Patel School, 1st N Block, 2nd Stage, Rajajinagar, Bengaluru 560010
The hospital is open 24 hours. Call before you come to check the timing for your scan or sample collection.
Get directions ↗IVF Pulse fertility care at Tamara Hospital →
Reception
Waiting area
IVF lab
This page explains each test in general terms so you know what to expect. It is not medical advice: your doctor decides which tests you need and what your results mean. Reference ranges differ between laboratories. Updated 4 November 2026.