Free T4 (Free Thyroxine)

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also ft4

What it isFree T4 is the fraction of thyroxine that isn't bound to transport proteins. The thyroid secretes T4 as the storage form; tissues take it up and convert it to active T3.

DoesProvides the substrate pool the body converts to active thyroid hormone wherever and whenever it's needed.

GoodFree T4 in the upper-mid normal range signals robust thyroid output and adequate substrate for tissue conversion.

BadLow Free T4 indicates hypothyroidism; high Free T4 indicates hyperthyroidism. Patterns that diverge from TSH expectations suggest pituitary or conversion problems.

Standard

0.8–1.8 ng/dL.

Optimal

1.2–1.6 ng/dL (narrow target)

Modifiers

Biotin supplements interfere with the assay; pregnancy alters reference ranges.

How to optimize

Lifestyle: Manage stress; sleep; avoid extreme caloric restriction.
Diet: Adequate iodine, selenium, zinc, iron, protein.
Supplements: Selenium (200µg/day), zinc (15–30mg/day), iron if deficient.
Peptides: —
Drugs: Levothyroxine for hypothyroidism.

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