Thyroxine (T4)
Thyroxine (T4)
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Description
Thyroxine (T4) and triiodothyronine (T3) are produced and stored in the thyroid gland, integrated into thyroglobulin. The majority of T4 and T3 in the body is bound to proteins, rendering them inactive. This test specifically measures the free, biologically active form of T4, which plays a critical role in regulating the body’s metabolism.
Levothyroxine (commonly prescribed as Levaxin or Euthyrox) is a synthetic form of T4 often prescribed to individuals with hypothyroidism to normalize T4 levels when the thyroid gland does not produce sufficient amounts on its own.
High Levels
Elevated free T4 levels are typically seen in cases of thyroid overactivity, known as hyperthyroidism. In patients undergoing treatment for hypothyroidism, values near the upper reference limit are often observed when treatment is effective. High levels of free T4 can result in symptoms such as anxiety, feelings of stress, palpitations, tremors, excessive sweating, general malaise, and unintended weight loss.
Low Levels
Underproduction of thyroid hormones is associated with reduced volume in the right hippocampus (1). Low levels of free T4 are usually observed in cases of thyroid underactivity, or hypothyroidism. In rare instances, low levels may occur due to circulating antibodies against T4. Low T4 levels can also be temporarily observed during the third trimester of pregnancy without indicating hypothyroidism. Symptoms of low free T4 include fatigue, depression, feeling cold, and weight gain.
