Free T3 (Free Triiodothyronine)

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also ft3

What it isFree T3 is the unbound, biologically active form of thyroid hormone. Most T3 is generated by removing an iodine atom from T4 in peripheral tissues; a small amount comes directly from the thyroid.

DoesBinds to nuclear receptors and drives metabolic rate, body temperature, heart rate, cognition, mood, and protein synthesis.

GoodFree T3 in the upper-mid range signals efficient T4-to-T3 conversion and intact tissue thyroid hormone action.

BadLow Free T3 with normal Free T4 suggests impaired conversion ("low T3 syndrome") and tracks fatigue, depression, weight gain, and cognitive sluggishness.

Standard

2.3–4.2 pg/mL.

Optimal

3.4–4.2 pg/mL (higher is good within range)

Modifiers

Falls with caloric restriction, illness, stress, selenium deficiency.

How to optimize

Lifestyle: Avoid extreme caloric restriction; resistance training; sleep; stress management.
Diet: Adequate calories; selenium-rich foods (Brazil nuts, sardines); zinc; iron.
Supplements: Selenium (200µg/day) — selenoenzymes drive T4-to-T3 conversion; zinc (15–30mg/day); iron if deficient.
Peptides: —
Drugs: Liothyronine (T3) supplementation in selected patients; combination T4/T3 therapy.

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