In-vitro Evaluation of Thyroid Function

SUMMARY The level of “free thyroxine” is currently considered to be the major determinant of thyroid status. It depends on the total thyroxine concentration in plasma and upon the unsaturated capacity of the plasma proteins for thyroxine. These can now be determined by a variety of different techniques which do not require the administration of radioactivity to the patient and which are unaffected by the presence of excess iodide. Since many of these procedures have been shown to be precise, yet simple to perform, these in vitro tests of thyroid function are proving of increasing value to the clinician for the confirmation of the clinical diagnosis of thyroid disease.