T3 and T4 are thyroid hormones that regulate metabolism and other body functions. Laboratory tests determine the level of these hormones in the body to help diagnose thyroid disorders.
The thyroid gland uses iodine to make thyroxine (T4) and triiodothyronine (T3). The pituitary gland releases thyroid stimulating hormone (TSH) to induce T4 and T3 production in the thyroid gland.
Common tests that measure the amount of T3 and T4 include blood tests that determine serum T3 and T4 levels and radioimmunoassay (RIA) for patients who do not take thyroid replacement medication.
According to the University of Michigan Health System, the normal range of T4 is 5–13.5 micrograms per decilitre. If the level of T4 is higher, then the thyroid is producing too much hormone or the dosage of thyroid replacement medication is too high. A lower level indicates decreased thyroid function or that the dosage of thyroid replacement medication is too low.
The normal range for T3 is 100-200 nanograms per decilitre, which equals 0.1-0.2 micrograms per decilitre. A higher level of T3 may indicate hyperthyroidism, Graves disease, or possibly thyroid cancer. Lower T3 may indicate hypothyroidism or Hashimoto’s disease. Liver disease and pregnancy cause artificially high T3 readings.
Oestrogen and some medications can affect the level of T3 and T4 in the blood. A combination of tests can more accurately identify thyroid abnormalities than a single test.