Triiodothyronine or T3 is the most biologically active thyroid hormone in humans. The term “free T3” means the amount of T3 that is not bound to proteins in the blood. T3 acts on almost every cell in the body by setting the metabolic rate of the cell. T3 is also critical for growth and development, especially in fetuses and children. The thyroid gland produces and releases some of the T3 in the blood, but about 80% of T3 is produced from T4 or thyroxine in the liver, kidney, and thyroid tissue. Thyroid-stimulating hormone, also known as TSH or thyrotropin is produced in the pituitary gland an stimulates the release of T3 by the thyroid gland. Most clinical laboratories do not directly measure of free T3. Instead, they measure total T3 and thyroid hormone-binding index.
Normal Ranges for Free T3:
1-23 Months 3.3-5.2 pg/mL
2-12 Years 3.3-4.8 pg/mL
13-20 Years 3.0-4.7 pg/mL
>20 Years 2.3-4.2 pg/mL
Decreased free T3 levels in the serum usually indicate hypothyroidism or chronic or subacute thyroiditis. Drugs that interfere with the conversion of T4 to T3 will decrease free T3 levels, and increase T4 levels. In most cases, free T3 is a less useful indicator of thyroid function than free T4. Free T3 may be normal in hypothyroidism, when TSH levels are high and free T4 levels are low. Any serious illness can decrease free 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
- Serious, non-thyroidal illness
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Elevated free T3 levels may indicate hyperthyroidism, thyroid hormone resistance syndrome, or T3 toxicosis. Serum T3 levels can be expected to increase during normal pregnancy; however, hyperthyroidism should be suspected if free T3 levels exceed normal values for a pregnant woman in a given trimester. Free T3 is generally more accurate in cases of hyperthyroidism than it is for diagnosing hypothyroidism. Elevated T3 may cause symptoms of hyperthyroidism including excessive appetite, anxiety, heart palpitation, shortness of breath, sweating, weight loss, emotional disturbances, and intolerance to heat.
Some specific causes of high free T3 are:
- Hyperthyroidism (e.g. Grave’s disease)
- Acute thyroiditis (e.g. postpartum thyroiditis)
- Pregnancy (normal, unless it exceeds pregnancy normal values)
- T3 toxicosis
- Thyroid hormone resistance syndrome
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