What do the results mean?Your results may come in the form of total T4, free T4, or a free T4 index.The free T4 index includes a formula that compares free and bound T4.High levels of any of these tests (total T4, free T4, or free T4 index) may indicate an overactive thyroid, also known as hyperthyroidism.Low levels of any of these tests (total T4, free T4, or free T4 index) may indicate an underactive thyroid, also known as hypothyroidism.If your T4 test results are not normal, your health care provider will likely order more thyroid tests to help make a diagnosis. These may include:T3 thyroid hormone tests. T3 is another hormone made by the thyroid.A TSH (thyroid stimulating hormone) test. TSH is a hormone made by the pituitary gland. It stimulates the thyroid to produce T4 and T3 hormones.Tests to diagnose Graves' disease, an autoimmune disease that causes hyperthyroidismTests to diagnose Hashimoto's thyroiditis, an autoimmune disease that causes hypothyroidismLearn more about laboratory tests, reference ranges, and understanding results.
