Triiodothyronine (T3) is the most biologically active thyroid hormone in humans. It is sometimes called total triiodothyronine because it includes both free triiodothyronine and triiodothyronine bound to proteins. Triiodothyronine acts on almost every cell in the body, binding to thyroid hormone receptors and setting the metabolic rate of cells. Triiodothyronine is critically important for the growth and development of fetuses, infants, and young children. While the thyroid gland produces triiodothyronine, about 80% is created from the other active thyroid hormone, thyroxine. More than 99% of triiodothyronine is bound to proteins in the serum, the rest is free triiodothyronine (free T3).
Normal Ranges for Total T3:
Decreased triiodothyronine levels in the serum usually indicate hypothyroidism or chronic or subacute thyroiditis. Any serious illness can decrease triiodothyronine levels. Abnormally low triiodothyronine may cause symptoms of hypothyroidism including weakness and fatigue, cold intolerance, shortness of breath, weight gain, constipation, cognitive problems, dry skin, hoarseness, and swelling (edema).
Some specific causes of low triiodothyronine are:
Elevated triiodothyronine levels may indicate hyperthyroidism, thyroid hormone resistance syndrome, or triiodothyronine toxicosis. Serum triiodothyronine levels can be expected to increase during normal pregnancy, but other tests may be necessary to rule out hyperthyroidism. Elevated triiodothyronine may cause symptoms of hyperthyroidism including excessive appetite, anxiety, heart palpitations, shortness of breath, sweating, weight loss, emotional disturbances, and intolerance to heat.
Some specific causes of high triiodothyronine are:
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