Endocrine Abstracts

Background: Euthyroid hyperthyroxinaemia can present a diagnostic challenge. Abnormalities in the binding proteins of thyroid hormones can cause this discordant picture of thyroid function tests, with thyroxine binding globulin being the protein most commonly affected. Familial dysalbuminaemic hyperthyroxinaemia is a rarer cause, and is an autosomal dominant condition which can present with euthyroid hyperthyroxinaemia. This condition is associated with a mutation in albumin which causes an increased affinity to thyroxine (T4) and triiodothyronine (T3) and hence interference in immunoassays.

Case: A 34 year old woman and her 65 year old mother were referred to endocrine clinic with an unusual pattern of thyroid function tests. They had no symptoms of thyroid dysfunction and neither woman had a goitre. The daughter had a normal thyroid stimulating hormone (TSH) with raised free T4 of 32 pmol/l (9 to 21), and the mother had a normal TSH with raised free T4 between 23 pmol/l and 25.5 pmol/l for the preceding 6 years. The sex hormone binding globulin was normal in both women. TSH was normal across multiple different platforms, however raised free T4 was reproduced on Roche and Siemens platforms, but not with an Abbott platform. The Abbott platform demonstrated normal TSH and normal free T4 for both mother and daughter. Results were unchanged following testing with blocking tube to remove heterophilic antibodies that could potentially cause interference. Familial dysalbuminaemic hyperthyroxinaemia was suggested as a likely diagnosis to explain thyroid function results, with an alternative diagnosis being a transthyretin mutation as opposed to albumin mutation. Serum has been sent for a radioligand binding assay and results are awaited.

Discussion: Knowledge of the potential causes of euthyroid hyperthyroxinaemia is important, as it has the potential to prevent unnecessary treatment of hyperthyroidism.