Measurement of Thyroid Hormone Levels	

	Assays that measure free T4 or free T3 directly are now available and are useful in the assessment of the thyroid status of a patient. The single-step assays probably function effectively as a combined measure of T4 and free hormone fraction (as does the FTI), and have the virtue of simplicity. Free thyroid hormone levels should not be affected by the concentration of normal thyroid hormone binding proteins.  
Indirect methods of estimating free thyroid hormone levels are widely used. These tests combine the determination of total serum T4 or T3 with an estimate of the free fraction to derive a free hormone index. Basically, the indirect methods ‘correct’ for the effect of altered binding proteins (primarily TBG), and have provided, for over three decades, a highly, although not perfectly, reliable estimate of free T3 or free T4.