Free T4 (FT4) - Free Thyroxine

Clinical definition of the Free Thyroxine (FT4) test
Thyroxin (T4) is one of the two major hormones (the other called Triiodothyronine) produced by the thyroid gland. In the blood, T4 is either free (not bound) or protein-bound (bound to thyroxin- binding globulin). The free form of T4 is biologically active, hence measuring concentrations of free thyroid  hormone is of diagnostic value. The T4 test measures the amount of thyroxin in the blood and helps control metabolism and growth.

Why is the Free Thyroxine (FT4) test done?
Free thyroxine test is carried out to evaluate thyroid function, to help diagnose hypothyroidism or hyperthyroidism and to screen for hypothyroidism in new borns and aid in the diagnosis of female infertility. This test is more accurate as it is not affected by protein level.

What are the common signs/symptoms when the Free Thyroxine (FT4) test is done?
Excessive amounts of T4 in the blood results in symptoms associated with hyperthyroidism such as:

  • Nervousness
  • Tremors of the hands
  • Weight loss
  • Insomnia
  • Puffiness around dry, irritated eyes
  • Increased heart rate
  • Anxiety

Insufficient amounts of T4, show symptoms associated with hypothyroidism and a slowed metabolism, such as:

  • Weight gain
  • Dry skin
  • Fatigue
  • Constipation
  • Irregular menstruation

What should I do before the Free Thyroxine (FT4) 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 Thyroxin (FT4) test results?

Reference Range*


0.8 to 1.8 ng/dL


‘*A Reference range is a set of values which helps the healthcare professional to interpret a medical test. It may vary with age, gender, and other factors. Reference ranges may also vary between labs, in value & units depending on instruments used and method of establishment of reference ranges’

Diseases/conditions related to the Free Thyroxine (FT4) test

  • Hypothyroidism
  • Hashimoto's disease
  • Graves disease
  • Germ cell tumors
  • Iodine-induced hyperthyroidism
  • Subacute or chronic thyroiditis
  • Toxic multinodular goiter
  • Trophoblastic disease
  • Goiter
  • Thyroid Disorders
  • Acute thyroiditis
  • Cretinism
  • Cirrhosis
  • Malnutrition
  • Hepatitis

Other tests related to the Free Thyroxine (FT4) test

  • TSH
  • T3
  • Thyroid Antibodies

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Free T4 (FT4)