Endocrine Abstracts

A 52-year-old patient with a history of secondary progressive multiple sclerosis was referred to our endocrine vetting service with deranged thyroid function tests (TFTs). Medicines reconciliation demonstrated that the patient was currently on regular fampridine and carbamazepine only. TFTs checked in the community revealed a free T4 of 65.8pmol/l (12.0-22.0pmol/l) with a TSH of 2.03mU/l (0.27-4.20mU/l) and a free T3 of 5pmol/l (3.1-6.8pmol/l). 25-OH vitamin D and vitamin B12 were raised above the reference range, prompting a discussion of whether this patient was taking any over the counter medications. It became apparent that they were taking regular multi-vitamin supplements containing vitamin B7 (biotin) which is known to interfere with the Roche immunoassay. With regards to competitive assays such as free T4, excess biotin competes with the antibody in the reagent for binding sites on streptavidin, causing falsely elevated results. TFTs were subsequently sent to a laboratory in a neighbouring health board and were analysed using the Abbott immunoassay and results returned euthyroid with a free T4 of 13.5pmol/l (9.0-21.0pmol/l) and TSH 2.56mU/l (0.3-5.0mU/l). Biotin is a water-soluble B-complex vitamin that has an important role in metabolism and cell-turnover, and is commonly found in multi-vitamins and dietary supplements. Most multi-vitamins will contain between 40-62.5 mg of biotin, greater than the recommended daily intake of 30 mg. The majority of adults will meet their daily requirements through diet alone, in foods such as eggs, dairy products, vegetables and meat. There have been studies recommending higher biotin doses for patients with demyelinating neurological conditions such as multiple sclerosis. Unfortunately, it is not known what daily dose of biotin our patient was receiving. This case highlights the importance of interpreting deranged thyroid function tests with caution, particularly when the patient is asymptomatic, as was the case here. Similar cases have resulted in patients receiving unnecessary treatment with carbimazole which risks potentially severe side effects. After discussion with our senior biochemists, this assay interference is well documented within the laboratory setting, but is less well known clinically. At the time of writing this case report, Roche are currently working on assays without this interference and we eagerly await the results of this patient’s thyroid function when assessed via the updated assay.