Biotin (Vitamin B7) is an essential co-factor for five carboxylases involved in fatty acid synthesis and energy production. The recommended daily intake (RDI) of biotin ranges from 30-70mcg per day (1).  At high doses (10,000 times RDI), biotin improves clinical outcomes and quality of life in patients with progressive multiple sclerosis(1). Biotin is a common component of multivitamin preparations and at moderate doses (100-100 times RDI) has been reported to cause interference in immunoassays resulting in abnormal thyroid function tests (TFTs). Biotin-related interference has also been described in other immunoassays including ferritin, oestradiol and thyroglobulin (2,3).
A 63 year old woman with secondarily progressive MS, treated with 300mg of biotin per day was referred due to markedly elevated thyroid function tests (FT4>100pmol/L [RR 7-16pmol/L], FT3 11.6pmol/L [3.6-6.5pmol/L], TSH<0.03mU/L [RR 0.3-4.2mU/L]). TFTs were normal 6 months prior to starting biotin and she had no clinical features of thyrotoxicosis. Serum B12 levels were >1400pmol/(170-800pmol/L) in the absence of supplementation.  Assay interference with biotin was suspected. Biotin was then stopped for 24 hours, repeat TFTs and B12 levels were conducted using two different platform immunoassays (Elecsys 2010 assay and Beckmann- Coulter Access). The latter assay  does not rely  on the biotin/streptavidin interaction. The patient’s samples were also tested for heterophile antibodies using the scantibody blocking tube method.
Heterophile antibodies were absent. Following biotin cessation the patient’s thyroid function tests using both assays returned to normal (TSH 1.93mU/L, T4 14pmol/L). Vitamin B12 levels were 184pmol/L on re-testing. Upon biotin rechallenge FT4 was 77.3pmol/L, TSH 2.58mU/L using the Beckmann- Coulter Access.
Biotin ingested in high doses causes assay interference for TFTs and B12. Interference can occur by multiple mechanisms depending on the assay format. Mechanisms to remove biotin such as a (streptavidin agarose column, non-biotinated assays) will minimise the impact of assay interference on clinical decision-making. Clinicians must be aware of immunoassay interference by biotin to avoid misdiagnosis.

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