Euthyroid hyperthyroxinaemia due to assay interference

Background: The authors report a case of euthyroid hyperthyroxinaemia and the systematic approach that led to the diagnosis. The related literature is also reviewed in an attempt to increase awareness of this condition. Case report: A 47-year-old female patient was referred for further investigation and management of "hyperthyroidism." The patient was clinically euthyroid and had previously been treated with carbimazole, but self-discontinued therapy as she felt unwell on treatment. A careful review of this patient's blood results revealed elevated free thyroxine and unsuppressed thyroid-stimulating hormone (TSH). This is atypical of primary hyperthyroidism, in which case suppressed TSH would have been expected. In view of the clinical euthyroidism, euthyroid hyperthyroxinaemia was considered the most likely diagnosis and an appropriate work-up was initiated. Following on the consultation with the Chemical Pathology Unit, assay interference was established as the likely cause and the patient was reassured. She remains well, with no treatment. Conclusion: Thyroid function tests should not be interpreted in isolation and, if the clinical picture and biochemistry are discordant, it is imperative to consider assay interference. It is also important to apply basic physiological principles in interpreting endocrine blood results. In this patient, both the clinical euthyroidism and the unsuppressed TSH, which are atypical of primary hyperthyroidism, prompted further work-up.