Higher-than-normal T3 levels indicate an overactive thyroid, liver disease, toxic nodular goiter, thyroid medications, supplement consumption, pregnancy or birth control pill usage, whereas lower-than-normal levels mean illness, thyroiditis, underactive thyroid or starvation, according to MedlinePlus. T3, otherwise known as triiodothyronine, is a thyroid hormone that helps control metabolism.
Certain medications increase T3 levels, including estrogens, birth control pills, methadone, some herbal remedies and clofibrate. MedlinePlus indicates medications that decrease T3 measurements include androgens, anabolic steroids, amiodarone, anti-thyroid drugs, lithium, propranolol and phenytoin.
A doctor measures T3 levels to determine thyroid gland function. Normal values range from 100 to 200 nanograms per deciliter of blood. MedlinePlus reveals laboratory tests measure T3 levels attached to proteins and free-floating in the bloodstream.
Healthline reveals doctors prescribe a T3 test if they suspect hyperthyroidism, hypopituitarism, hypothyroidism and thyrotoxic periodic paralysis. Thyroid disorders cause many symptoms, since the thyroid gland affects several body functions. Some symptoms of thyroid dysfunction include anxiety, constipation, irregular menstruation, weakness, fatigue, difficulty sleeping, increased sensitivity to heat or cold, hair loss and weight changes.
Doctors also order tests of thyroxine, or T4, and thyroid stimulating hormone, or TSH, to determine levels of other thyroid hormones. Healthline states endocrinologists get a better idea of what happens in the thyroid when all three main thyroid hormones are measured.