What the Free T4 Test (FT4) measures
Free T4 (free thyroxine) measures the part of the T4 hormone in the blood that is not bound to proteins and can enter the body’s cells. T4 is the main hormone released by the thyroid gland, and the body converts it into the more active T3 as needed. Most T4 travels attached to carrier proteins, and the level of those proteins changes in pregnancy, with oestrogen-containing pills and in some illnesses. Free T4 is less affected by those changes than total T4, which is why many doctors prefer it. Free T4 is usually read together with TSH. When TSH is high, a low free T4 points towards an underactive thyroid (hypothyroidism), while a normal free T4 suggests a milder, subclinical change. When TSH is low, a raised free T4 points towards an overactive thyroid (hyperthyroidism). Free T4 is also used to adjust the dose of thyroxine tablets, during pregnancy, and in the less common situations where the pituitary gland, rather than the thyroid, is the cause of the problem, because TSH alone can be misleading then. Some medicines and severe illness can affect the result, so it is read alongside your symptoms and other tests.
Price: the Free T4 Test (FT4) costs ₹280 at Pure Bliss (MRP ₹350), with the sample collected at our Action Area 2, New Town centre or at your home.
Who this test is for
- An abnormal TSH result that needs follow-up
- Symptoms of an underactive or overactive thyroid
- Pregnancy, or planning a pregnancy, when the doctor advises
- People on thyroid tablets whose dose is being adjusted
How to prepare, the sample, and when to expect the report
Preparation
Fasting is not required. A morning sample is preferred. If you take thyroxine, give the sample first and take the tablet afterwards, as a dose taken shortly before can raise the reading. Stop biotin supplements for 48 hours beforehand.
Sample
Blood, from a vein in the arm (serum).
Reporting time
Within 24 hours.
Reading the result
Free T4 ranges differ between laboratories and methods, and pregnancy has its own ranges. The value is usually read together with TSH. Reference ranges differ between laboratories, so your report lists the laboratory’s own range beside each value. A result outside the range does not always mean illness; ask your doctor to explain it.
- Centre or home collection
- NABL-accredited Redcliffe Labs collection centre
- How collection works →
The Free T4 Test (FT4) can be booked at the clinic in Action Area 2 or with a home collection from 7:30 AM. Patients come to us from Action Area 1, Action Area 3, Rajarhat, Chinar Park, Salt Lake Sector V, Sukhobrishti and Eco Park.