Principles and pitfalls of free hormone measurements.

The free hormone hypothesis states that a hormone's physiological effects depend on the free hormone concentration, not the total hormone concentration. Although the in vivo relationship between free hormone and protein-bound hormone is complex, most experts have applied this view to the design of assays used to assess the free hormone concentration in the blood sampled for testing in vitro. The history of the measurement of free thyroxine, probably the most frequently requested free hormone determination, offers a good example of the approaches that have been taken. Methods that require physical separation of the free hormone from the protein-bound hormone must address both the potential disturbance in the equilibrium between the two, as well as the challenge of quantifying small levels of hormone accurately and precisely. The implementation of mass spectrometry in the clinical laboratory has helped to develop proposed reference measurement procedures. These must be utilized to standardize the variety of immunoassay approaches that currently represent options commercially available to the routine clinical laboratory. Practicing endocrinologists should discuss the details of the free hormone assays offered by the clinical laboratory they utilize for patient result reporting, and clinical laboratories should implement the recommendations of published guidelines to ensure that free hormone results using commercially available immunoassays are as accurate and precise as possible.