At a glance
| Analyte | Free thyroxine |
|---|---|
| Also called | Free T4, Free thyroxine |
| Method | Competitive chemiluminescence immunoassay |
| Sample type | Serum, Plasma |
| Panel | Thyroid |
| Discipline | CLIA immunoassay |
| Brand | Mindray |
What FT4 measures
Thyroxine circulates almost entirely bound to carrier proteins, and only the small unbound fraction is available to act on tissue. FT4 measures that free fraction, which is why it has largely replaced total T4 in routine practice: the total moves whenever the carrier proteins move, and carrier protein levels change in pregnancy and with oestrogen therapy without the patient’s thyroid status changing at all. Read with TSH, FT4 separates the common patterns. A high TSH with a low FT4 indicates an underactive thyroid; a suppressed TSH with a raised FT4 indicates an overactive one; and a pattern where the two move together rather than in opposite directions points away from the thyroid itself and toward the pituitary.
How Philippine laboratories run it
FT4 is added to a thyroid request either from the outset or reflexed when the TSH falls outside the laboratory interval, so its volume tracks TSH closely but sits below it, and the two should be scheduled together with that ratio in mind. Antenatal clinics are a significant driver: thyroid status is monitored in pregnancy, and the shift in carrier proteins that pregnancy causes is precisely the reason the free measurement is used rather than the total. Laboratories serving obstetric services should expect FT4 demand to hold up even when general outpatient volume dips.
Analyzers it runs on
Calibrator, controls and companion tests
Delivered with the control material it is verified against and the rest of the thyroid panel, on the same schedule as FT4.
Storage and handling
Refrigerated, upright, and not frozen.
Buying FT4 in the Philippines
FT4 is supplied for the CL-2600i and CL-900i. We deliver it with Thyroid Control and TSH, because a run held up waiting on a separate order costs the same shift a stock-out does. Delivery is scheduled against your monthly FT4 count, so the bench is not rationing tests in the last week of the month. How we plan clia immunoassay supply.
Questions about FT4
Why measure free T4 rather than total T4?
Because only the unbound fraction is biologically active, and the total is distorted by anything that changes the carrier proteins. Pregnancy and oestrogen therapy both raise carrier protein levels, which raises a total T4 without the patient being thyrotoxic. The free measurement is unaffected by that, which is why it has become the routine test.
Order FT4
Tell us your monthly FT4 volume and we will quote a supply schedule for CL-2600i. We reply within one business day.
Request a quotation