Range of result differences of thyroid function tests

Range of result differences of thyroid function tests on aliquots of the same sample, measured with 15 different immunoassays. These sample levels of thyroid hormones and thyroid hormone-binding proteins are within the normal reference range.

In samples with abnormal concentrations of thyroid hormones or thyroid hormone-binding proteins, even larger differences are observed between the assays for free thyroxine (FT-4) and free triiodothyronine (FT-3). TT-4 indicates total thyroxine; TT-3, total triiodothyronine.