The interference of
familial dysalbuminemic hyperthyroxinemia (FDH), antithyroxine (anti-T4) or antitriiodothyronine (anti-T3)
antibodies in serum free T4 (FT4) or free T3 (FT3) assays depends on the assay method: two-step immunoextraction, one-step labelled
ligand (analogue or derivative tracer) or labelled antibody (solid-phase
antigen-linked technique: SPALT) method. For FDH subjects, only FT4 measurements by one two-step method gave reliable results in all cases. However, the overestimation was less marked with new SPALT assays than with previous analogue-based assays. In sera containing anti-T4
antibodies, two-step assays and SPALT assays with a T3-coated solid phase can be considered as reliable for FT4 determination. All other methods including SPALT assays with a T4-coated solid phase may give falsely high results. In sera containing anti-T3
antibodies, SPALT FT4 and FT3 assays with a T3-coated solid phase may give spuriously high values. Anti-T3
antibodies do not interfere in two-step FT3 assays and SPALT FT3 assays with a T2-coated solid phase, but may give high FT3 values as measured by analogue or derivative methods. Moreover, SPALT free
thyroid hormone assays may also be subject to interference from
antibodies directed towards the assay solid phase.