MedlinePlus reports that normal T4 test levels range from 4.5 to 11.2 micrograms per deciliter; however, some medications can affect test results. A T4 test shows how much thyroxine is present in a blood sample.
According to MedlinePlus, thyroxine is the primary hormone produced by the thyroid gland, so a blood sample test is a good indicator of how well the thyroid is functioning. Some medications increase T4 levels, including birth control pills, estrogens and methadone, while amiodarone, barbituates and lithium are among numerous drugs that decrease levels.
MedlinePlus explains that high T4 levels can indicate one of several conditions associated with an overactive thyroid. Lower levels are associated with hypothyroidism, illness or malnutrition. Some laboratories use different methods or measurements, so a doctor should explain disparities among results.