Total T4
What it measures, and what changes the number.
Total T4 measures thyroxine, the main hormone released by the thyroid gland. Most thyroxine in the blood is carried bound to proteins such as thyroxine-binding globulin, and this test counts both the bound and the small unbound portion together.
It is usually read alongside TSH, which reflects the pituitary signal to the thyroid, and Free T4, which estimates only the unbound hormone. Total T3 or Free T3 may be added to see the more active hormone, and thyroid antibody tests are often ordered in the same panel.
Because the result includes protein-bound hormone, anything that changes binding protein levels, such as pregnancy or estrogen-containing medicines, can shift the number without a change in unbound hormone. Biotin supplements can interfere with some immunoassays. Labs may print it as T4 (THYROXINE), TOTAL or Tiroxina (T4).
Often read alongside: TSH, Free T4, Total T3, Free T3, Anti-TPO antibodies