FT4 and FT3 on a report: the free thyroid hormones

Bound and free

The thyroid makes two main hormones: T4, the bulk product, and T3, the more active one. In the bloodstream most of both travels attached to carrier proteins, like cargo on a ship. Only the small unbound — free — fraction can leave the blood, enter cells, and do the work.

This is why reports distinguish total from free. Total T4 or T3 counts everything, cargo included; FT4 and FT3 count only the working fraction. Because the carrier proteins themselves rise and fall — in pregnancy, for example — the free measurement often reflects the gland's true output more faithfully than the total.

When each is ordered

Free T4 is the standard second step after an abnormal TSH: high TSH with low FT4 confirms an underactive thyroid; low TSH with high FT4 confirms an overactive one. The pair separates clear dysfunction from the subclinical in-between, where TSH is off but FT4 still sits in range.

T3 measurements join mainly when an overactive thyroid is suspected — in that condition T3 can rise earlier and more sharply than T4. For an underactive gland, T3 adds little, because the body converts T4 to T3 as needed until late in the disease.

Reading the trio together

TSH, FT4, and FT3 form one logical chain: the pituitary signal, the main product, and the active amplifier. The pattern across them — not any single value — describes the gland. A flagged value is followed by the rest of the chain and, when a cause is sought, by antibody tests.

The familiar cautions apply: biotin supplements can distort thyroid measurements and are usually paused before the draw, and the laboratory's own reference ranges are the only valid yardstick for each line.

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