Hormonal production of the thyroid gland is constituted of
thyroxine or T4 (80%) and
triiodothyronine or T3 (20%). In the circulation, whole T4 originates from thyroid secretion but most of T3 (80%) is produced extrathyroidally from T4 deiodination. Conversion of T4 to T3 may be influenced by various conditions and circulating T3 is a less reliable reflection of
thyroid hormone production than T4. In serum most of T4 and T3 is bound to
binding proteins and only 0.02% of T4 and 0.3% of T3 is free. Because of their higher diagnostic performance, free T4 (FT4) and free T3 (FT3) measurements have superseded total (free + bound)
hormone determination. Total
hormone measurements remain useful for research studies or in case of severe
hyperthyroidism. Equilibrium dialysis/RIA is considered as the reference method for free
hormone measurements. Routine clinical laboratories use automated direct two-step or one-step immunoassays with a high molecular weight
ligand or labelled antibody. Free
hormone measurement remains technically demanding, especially in sera from severe non-thyroid ill patients with low serum
thyroxine binding capacity. Interference from anti-
thyroid hormone antibodies and
familial dysalbuminemic hyperthyroxinemia depends on the assay method, but is now less marked and less frequently detected. To be able to correctly interpret the results of an assay, it is necessary to assess its performance in biologically and clinically well-characterised serum samples. FT4, and FT3 measurements, if FT4 is normal and
hyperthyroidism suspected, are used to confirm and assess the level of hypo and
hyperthyroidism (overt or subclinical). When the thyroidal status is unstable (first months of a thyroid treatment, altered L-T4 dose,
subacute thyroiditis) or when the hypothalamic-pituitary function is disturbed (
central hypothyroidism), TSH determination is diagnostically misleading and only free
hormone measurements are reliable for thyroid function assessment.