In a recent issue of Thyroid, Barbesino reported on the interference of biotin intake in thyrotropin (TSH), free thyroxine (fT4), and triiodothyronine (T3) measurements, leading to a misdiagnosis of Graves’ disease (1). Other recent case reports have confirmed this finding (2, 3). Biotin has a high affinity for streptavidin, and this phenomenon has been used in the design of competitive (fT4, T3) and immunometric assays (TSH). In both types of assays, an excess of biotin can result in low signal production, although the result depends on the assay design: falsely high levels are observed in competitive immunoassays, such as fT4 and T3, and falsely low levels are observed in immunometric ‘‘sandwich’’assays, such as those used for TSH measurements (4, 5). To investigate how biotin directly interferes with these assays, samples were collected for TSH, fT4, and T3 measurements from 19 adult volunteers before and after oral ingestion of biotin (10 mg). The dose used was approximately 100 times higher than the required daily dose (30–100lg) but corresponds to the dose in commercial dietary supplements. Blood samples were collected before and at 3 and 24h after biotin ingestion. All the subjects collected the basal samples and the 24h samples between 7: 00 and 8: 00am at fasting, and 3h samples between 10: 00 and 11: 00am. None of the volunteers was taking any type of medication or had a personal or family history of thyroid disease. TSH and T3 measurement were performed with an electrochemiluminescence immunoassay (Roche Diagnostics GmbH, Mannheim, Germany; reference range 0.45–4.5 mIU/L for TSH and 70–200 ng/dL for T3). The fT4 measurement was performed with a chemiluminescent immunoassay (Access; Beckman Coulter, Brea, CA; reference range 0.6–1.3 ng/dL). TSH and T3 assays used streptavidincoated microparticles as the strategy for streptavidin-biotin interaction, and the fT4 assay used streptavidin-coated solid phase. After obtaining Fleury Ethics Committee approval (protocol number 1546784), signed informed consent was obtained from all patients.

The mean level of TSH was 2.84±1.27 mIU/L before biotin intake, and this decreased to 1.66±0.6 mIU/L at 3 h after biotin ingestion (p< 0.005). However, none of the subjects had a TSH below the lower reference range 3 h after biotin ingestion. One important aspect that should be considered is the failure in the diagnosis of subclinical hypothyroidism in the two subjects with slight elevation of basal TSH concentrations, which dropped, to normal values after biotin ingestion. Regarding fT4 measurements, the mean level of fT4 was 0.8±0.1 ng/dL before biotin ingestion and increased to