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Free T4 + Free T3

The thyroid is a small gland located just below the Adam’s apple that produces hormones to manage your body temperature, energy use, weight, muscle strength, and mood. These hormones are: T3 (triiodothyronine), which contributes to the body’s muscle control, digestion, heart function, and bone health; and T4 (thyroxine), which helps maintain metabolism, mood, and body temperature. Your body can convert T4 to T3 if necessary.

Healthcare professionals use free T3 and T4 levels to assess the functioning of your thyroid and pituitary glands. The pituitary gland is in the brain and manages the production and release of thyroid hormones.

You might experience anxiety, weight loss, and difficulty sleeping if your free T3 and T4 levels are elevated. These symptoms are associated with an overactive thyroid, which is also called hyperthyroidism. Hyperthyroidism is most commonly caused by Graves’ disease, an autoimmune disorder that causes the immune system to signal the thyroid to produce excess T4 and T3.

Those with a low level of free T3 and T4 might experience hair loss, weight gain, and tiredness. These symptoms are associated with an underactive thyroid, also known as hypothyroidism. Hypothyroidism is most commonly caused by Hashimoto’s thyroiditis, an autoimmune disorder that leads to chronic inflammation of the thyroid gland, causing it to produce lower quantities of hormones. 

This test may also be used to help evaluate the effectiveness of treatments taken to address a thyroid disorder.


Last updated:
August 6, 2021

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