MON-480 Isolated Hyperthyroxinemia - Does Everyone Needs Treatment?

Background: Raised free thyroxine (T4) with normal thyroid stimulating hormone (TSH) levels should be identified and interpreted with caution. Some of these conditions do not need treatment. We present three cases with similar biochemical abnormalities from three different causes. Case 1: A 62-year-old clinically asymptomatic lady was referred to us with Free T4 34.9 pmol/L (10.0 – 24.0 pmol/L), TSH 0.81 mU/L (0.2 – 5.0 mu/L) and negative TSH receptor antibodies (<0.9 IU/L). She was trialled on antithyroid drugs for 6 months. Her Free T4 stayed elevated between 29.0 – 35.0 pmol/L with normal TSH. We worked up for assay interference by running tests on two analysers, Roche Cobas e801 and Siemens ADIVA Centaur CP, both yielded similar results. Alpha1 glycoprotein subunits and SHBG were normal with clinical euthyroid status making TSHoma less likely. Serum protein electrophoresis did not detect any abnormal albumin. We were unable to perform equilibrium dialysis due to non-availability of facility at our centre. Due to strong clinical suspicion and family history of thyroid dysfunction that never needed a treatment, we tested her genetically for familial dysalbumineic hyperthyroxinemia (FDH) using mutation surveyor and fluorescent sequence analysis showed her to be heterozygous for c.725G>A ALB variant confirming diagnosis of FDH. Case 2: A 65-year-old clinically asymptomatic lady, was referred to us with Free T4 28.8 pmol/L (10.0 – 24.0 pmol/L) and TSH 2.50 mU/L (0.2 – 5.0 mu/L). Given inappropriately normal TSH levels, we repeated her TFTs using 3 different analysers, Roche cobas e801, Siemens ADIVA centaur CP and Abbot ARCHITECT i1000SR. Roche and Siemens assays yielded similar results, however Abbot assay showed normal thyroid function tests with TSH 1.01 mu/L (0.4-5.0 mu/L) and free T4 18.7pmol/L (9.0-19.0 pmol/L), confirming assay interference. As Siemens and Roche uses streptavidin-biotin immobilizing system while Abbot uses a magnetic bead-based capture system, the abnormal results could be due to biotin interference. Case 3: A 65-year-old lady, clinically asymptomatic was referred to us with Free T4 29.2 pmol/L (10.0 – 24.0 pmol/L) and TSH 1.59 mU/L (0.2 – 5.0 mu/L), 3 months after stopping amiodarone, which she took for 3 weeks for atrial fibrillation. This was thought to be due to amiodarone, owing to its long half-life of 58 days. We repeated thyroid function tests in 3 months from first clinical encounter i.e. 6 months after stopping amiodarone that showed Free T4 24.2pmol/L and TSH 2.30 mU/L and repeated further 3 months later that were normal, confirming amiodarone induced abnormal biochemical profile requiring no treatment. Conclusion: Hyperthyroxinaemia with normal TSH need to be interpreted with caution as illustrated above. Some of them do not need treatment and inappropriate interpretation can potentially cause anxiety for the patient and harm due to unnecessary treatment.