Thyroid Function Tests in Patients with Familial Dysalbuminaemic Hyperthyroxinaemia (FDH)

Two patients with familial dysalbuminaemic hyperthyroxinaemia (FDH) are described in whom the albumin variant resulted in raised total T4 levels, and artefactually raised free T4 using a 'single-step' technique employing an analogue of T4 as tracer. The first patient was clinically euthyroid and presented with relapse of schizophrenia and abnormal thyroid function tests (total T4 336 nmol/L, total T3 4.2 nmol/L, TSH 1.8 mU/L, free T4 73 pmol/L). These results led to diagnostic confusion and the patient was treated with a short course of anti-thyroid drugs. The second patient had signs and symptoms of thyrotoxicosis at her first visit but was clinically euthyroid 5 months later when she was 10 weeks pregnant. Thyroid function tests were total T4 259 nmol/L, total T3 3.6 nmol/L, TSH 3.8 mU/L, free T4 46 pmol/L. Further studies showed both patients to be biochemically euthyroid. A variant albumin was confirmed in both patients by a screening test for FDH and by reverse-flow electrophoresis. Family studies on 10 relatives of the first patient identified eight with FDH. A simple screening procedure for the indentification of FDH is described and the use of laboratory tests in suspected cases is discussed.