Endocrine Abstracts

Introduction: Biotin is a water-soluble vitamin involved in several and important metabolic processes, recommended daily intake for adults is 30–100 μg/day and normal plasma concentrations are 0.4–1.2 ng/day. Nowadays biotin is being used in inherited metabolic diseases, multiple sclerosis and as a supplement in, over the counter multivitamins widely used for common hair and skin problems. The supraphysiologic biotin intake has produced emerging problems with biotin-based immunoassays, producing interferences in different assays.

Case report: Our first case was a 55-year-old woman with progressive multiple sclerosis (PMS) referred to our Endocrinology Unit because a screening thyroid function test revealed a severe biochemical hyperthyroidism with TSH: 0.01 mUI/L (0.27–4.20 mU/L), FT4 >7.7 ng/dL (0.93–1.7 ng/dL), FT3: 10.9 pg/mL (2.0–4.4 pg/mL). The patient was admitted and methimazole with propranolol treatment was initiated, although clinical signs of thyroid disorder were unspecific and not clear, which made us doubt and continue with further investigations. Thyroid function tests were repeated and similar results were obtained with negative anti–thyrotropin receptor antibodies. Then we found that patient was using biotin 300 mg/d, therefore, an interference was suspected. When biotin was measured revealed a plasma concentration of 3830 ng/mL, (more than 1000-fold of normal concentration). The patient sample was also quantified by a biotin-free method and results confirmed biotin interference in our patient. Consequently, we tried to find a practical solution and applied a simple method described by Piketti and cols, to overcome the biotin interference. In addition, we designed an in vitro approach by adding biotin to a control serum-pool (with euthyroid status) increasing its concentration up to 1150 ng/mL simulating biotin intakes between 2.5–300 mg per day.

Conclusion: We confirmed that in competitive immunoassays [FT4 and FT3] excess biotin in the specimen competes with the biotinylated analogue for the binding sites on streptavidin, resulting in falsely high values. In contrast, when the sandwich immunoassay formats are used (TSH determination) excess of biotin in the sample displaces biotinylated antibodies, resulting in falsely low results. Therefore, the streptavidin-based method described, successfully eliminates biotin from de samples, avoiding false hyperthyroidism.