Rare coincidence: Macro-thyroid-stimulating hormone and multiple manufacturer-specific interferences in thyroid hormone immunoassays

Immunoassays are widely used for laboratory assessment of endocrine functions including thyroid hormones. While usually adequate for patient evaluation, they are known to potentially suffer from interference from a variety of factors. We report the case of a 44 year-old male patient without clinical symptoms of thyroid disease who presented for specialist evaluation after pathological thyroid function tests prompted a transferal by his primary care practitioner. Thyroid function tests showed discrepant results across immunoassays and platforms of different manufacturers. Polyethylene glycol precipitation prompted the diagnosis of macro-thyroid-stimulating hormone, while heterophilic and non-specific antibody blocking reagents proved ineffective in eliminating the interference in thyroid-stimulating hormone, free triiodothyronine and free thyroxine measurements. Further assessment ruled out a diagnosis of familial dysalbuminemic hyperthyroxinemia, leaving an exclusion diagnosis of manufacturer-specific interference in free triiodothyronine and free thyroxine assays due to unknown factors. Both clinicians and laboratory specialists must be aware of potential interference in immunoassays which otherwise might be misleading, potentially triggering unnecessary (invasive) follow-up procedures or therapeutic interventions. Close communication is required for successful troubleshooting. To our knowledge, no other case of both macro-thyroid-stimulating hormone and manufacturer-specific interference in a single patient has been documented thus far.