What is free T4 thyroid hormone

Thyroid gland is a small gland in front of your neck below the Adam’s apple and produces 2 thyroid hormones as T3 and T4. T4- thyroxine and T3- triiodothyronine  are produced by thyroid gland by using iodine.

Thyroid hormones T4 and T3 have 2 forms as free and bonded.  Big part of the thyroid hormones are bonded and transported by thyroid hormone-binding proteins like thyroxine-binding  globulin (TGB) or albumin. Free T4 travels the body and enters the tissues but bound T4 can’t enter to the tissues.

99% of the T4-thyroid hormone is found bonded and the rest part is free. 99% of the T4 is not in active form because T4 is converted to T3 hormone for body usage because T3 is the more active thyroid hormone in your body.

Thyroid hormones T4 and T3 regulates metabolism, heart rate, body temperature, carbohydrate metabolism, baby growth and menstruation. Thyroid hormones are so important in metabolic balance.

Free T4 blood test

free t4

Free T4 thyroid hormone blood test is done to evaluate thyroid gland functions. Free T4 blood test is the second test for thyroid evaluation after TSH test. TSH is the thyroid stimulating hormone, secreted by pituitary gland and stimulates thyroid gland for thyroid hormone production.  Free T4 blood test is done to diagnose hypothyroidism and hyperthyroidism conditions.

Free T4 normal levels

  • Adult: 0.58-1.64 ng/dL
  • Pregnants first trimester: 0.73-1.13 ng/dL
  • Pregnants second trimester: 0.54-1.18 ng/dL
  • Pregnants third trimester: 0.56-1.09 ng/dL


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