Free T3 (FT3) - Free Triiodothyronine

Clinical definition of the Free Triiodothyronine (FT3) test
Triiodothyronine (T3) is a thyroid hormone bound to carrier proteins that circulates in blood. The main transport protein is thyroxin binding globulin (TBG). The free (unbound) portion of triiodothyronine (free T3) is responsible for the biological action and plays an important role in the body’s control of metabolism. Free Triiodothyronine test measures the amount of T3 in the blood. Blood levels of T3 is increased or decreased because of insufficient or excessive production of the hormone by the thyroid gland due to thyroid dysfunction or pituitary dysfunction. Abnormal levels of T3 in the blood are associated with hyperthyroidism, hypothyroidism, and slow metabolism.

Why is the free triiodothyronine (FT3) test done?
FT3 is carried out to diagnose hyperthyroidism, to determine thyroid function and to help monitor the progress of a person with a thyroid disorder.

What are the common signs/symptoms when the free triiodothyronine (FT3) test is done?
Excessive amounts of T3 in the blood results in symptoms associated with hyperthyroidism such as:

  • Nervousness
  • Tremors of the hands
  • Weight loss
  • Insomnia
  • Puffiness around dry, irritated eyes
  • Eyes cannot move normally

Insufficient amounts of T3, show symptoms associated with hypothyroidism and a slowed metabolism, such as:

  • Weight gain
  • Dry skin
  • Fatigue
  • Constipation

What should I do before the Free Triiodothyronine (FT3) test?

Specimen type

Specimen collection procedure

Preparatory instructions before the test*


(Blood Sample)


(Collection of blood from a vein, usually from the arm)

No Fasting required.

No other special preparations required.

*Subjects suffering from any illness or on oral or injectable medications are advised to consult their physician prior to requesting any tests or procedures.

How do I interpret my Free Triiodothyronine (FT3) test results?

Reference Range*


1.4 to 4.4 pg/ml


* Reference ranges may vary between labs depending on instruments used and method of establishment of reference ranges’’

Diseases/conditions related to the Free Triiodothyronine (FT3) test

  • Graves’ disease
  • Hypopituitarism
  • Hypothyroidism - primary
  • Hypothyroidism - secondary
  • Thyrotoxic periodic paralysis
  • T3 thyrotoxicosis (Rare)
  • Thyroid cancer

Other tests related to the free Triiodothyronine (FT3) test

  • TSH
  • T4
  • Thyroid Antibodies
  • Painless (silent) thyroiditis
  • Thyrotoxic periodic paralysis
  • Toxic nodular goiter

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Free T3 (FT3)