Familial Dysalbuminemic Hyperthyroxinemia and Thyroid Hormone Autoantibodies: Interference in Current Free Thyroid Hormone Assays

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.