What is FT4?
A free thyroxine test (FT4), often called a free T4 test, measures the amount of free thyroxine in your blood. T4 circulates in the blood in two forms: free form (FT4) and bound form. Thyroxine (T4) is a hormone secreted by the thyroid gland and is essential for growth and metabolism. The FT4 test helps to diagnose any suspected thyroid disease (hypothyroidism or hyperthyroidism) and to check the treatment efficacy in people diagnosed with thyroid condition.

Higher T4 levels could indicate an overactive thyroid (hyperthyroidism) and shows symptoms like excessive appetite, anxiety, heart palpitations, sweating, shortness of breath, and weight loss. Abnormally low thyroxine level leads to hypothyroidism and causes symptoms like weakness, cold intolerance, weight gain, and dry skin.

Why is FT4 done?

The FT4 Test is done:

  • To diagnose any suspected thyroid disease (Hypothyroidism or Hyperthyroidism)
  • To monitor treatment in patients with thyroid diseases
  • What does FT4 Measure?
  • The thyroid gland secretes the following hormones:
  • Triiodothyronine (T3)
  • Thyroxine (T4)

Thyroid Stimulating Hormone (TSH) is a hormone secreted into the blood by the pituitary gland (a gland present in the brain) which tells your thyroid gland to make and release the thyroid hormones (T3 & T4) into your blood. The thyroid gland uses the iodine gained from food to make the thyroid hormones.

The thyroid hormones are essential for growth and metabolism. If the thyroid gland produces very high amounts of T3 and T4 hormones, you may experience symptoms like weight loss, rapid heartbeat, tremors, sweating, anxiety, increased sensitivity to heat, etc. This is known as hyperthyroidism.

The decreased production of thyroid hormones (T3 and T4) results in hypothyroidism which may cause weight gain, fatigue, slow heart rate, increased sensitivity to cold, depression, dry and thin hair, etc.

There is a feedback system in the body to maintain stable amounts of the thyroid hormones (T3 and T4) in the blood. When the levels of thyroid hormones decrease, the pituitary gland is stimulated to release TSH. High TSH, in turn, increases the release of thyroid hormones (T3 & T4) from the thyroid gland and vice-versa.

T4 hormone constitutes about 90% of thyroid hormones and circulates in the blood in two forms:

  1. Bound form – It is bound to the proteins present in the blood and this prevents it from entering the body tissues. The three main proteins in the blood that the T4 hormone is bound to are albumin, transthyretin and Thyroxine-binding globulin (TBG), also called Thyroid hormone binding globulin (THBG).
  2. Free form – This is the active form which enters the body tissues where it’s needed. Free Thyroxine (FT4) constitutes only 0.3% of the total T3 hormone.

Hence, the T4 hormone can be measured as Free T4 or Total T4. Total T4 includes both the bound and the free forms, circulating in the blood and can be affected by the amount of protein available in the blood to bind to them. Therefore, Thyroxine (T4) Free Test is a useful indicator of the T4 levels in the blood when binding proteins are increased or decreased.

Thyroxine (T4) Free Test is also done as a part of the Thyroid Profile Free Test which includes two more tests: Thyroid Stimulating Hormone (TSH) and Triiodothyronine (T3) Total.

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