Thyroxine (T4) TEST
T4 is the main form of thyroid hormone circulating in the blood. A Total T4 measures the bound and free hormone and can change when binding proteins differ (see above). A Free T4 measures what is not bound and able to enter and affect the body tissues. Tests measuring free T4 – either a free T4 (FT4) or free T4 index (FTI) – more accurately reflect how the thyroid gland is functioning when checked with a TSH.
The finding of an elevated TSH and low FT4 or FTI indicates primary hypothyroidism due to disease in the thyroid gland. A low TSH and low FT4 or FTI indicates hypothyroidism due to a problem involving the pituitary gland. A low TSH with an elevated FT4 or FTI is found in individuals who have hyperthyroidism.