Triiodothyronine or T3 is the most biologically active thyroid hormone in humans. It is called total T3 because it includes both free T3 and T3 bound to proteins. T3 acts on almost every cell in the body by setting the metabolic rate of cells. T3 is critically important for growth and development, especially in fetuses and children. The thyroid gland produces and releases T3 into the blood; however, four-fifths of T3 is formed by the conversion of another thyroid hormone T4 (thyroxine). This takes place in the liver, kidney, thyroid, and other tissues. Free T3 is the biologically active portion of total T3. Approximately 99.5% of T3 in the serum is bound to serum proteins such as thyroxine binding globulin (TBG).
Normal Ranges for Total T3:
Adult (> or =20 years): 80-200 ng/dL
0-5 days: 73-288 ng/dL
6 days-2 months: 80-275 ng/dL
3-11 months: 86-265 ng/dL
1-5 years: 92-248 ng/dL
6-10 years: 93-231 ng/dL
11-19 years: 91-218 ng/dL
Decreased T3 levels in the serum usually indicate hypothyroidism or chronic or subacute thyroiditis. Any serious illness can decrease T3 levels. Abnormally low T3 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 free T3 are:
- Chronic thyroiditis
- Subacute thyroiditis
- Nephrotic syndrome
- Drugs that decrease TBG (e.g. anabolic steroids, glucocorticoids)
- High doses of biotin may cause interference with this marker
Elevated T3 levels may indicate hyperthyroidism, thyroid hormone resistance syndrome, or T3 toxicosis. Serum T3 levels can be expected to increase during normal pregnancy, but other tests may be necessary to rule out hyperthyroidism. Elevated T3 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 free T3 are:
- Euthyroid hyperthyroxinemia
- T3 toxicosis
- Acute thyroiditis
- Familial dysalbuminemic hyperthyroxinemia
- Drugs that increase TBG (e.g. estrogens, tamoxifen, opioids)
- Synthetic T3 treatment
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