Erroneous thyroid diagnosis due to over-the-counter biotin.

OBJECTIVES: Biotin is a component of the vitamin B complex used in standard immunoassays to detect serum levels of various hormones and non-hormones, including thyrotropin (TSH) and thyroxine. These assays involve a strong bond between streptavidin and biotin, which serves as an anchor for measured analytes. Large doses of exogenous biotin for the treatment of certain medical conditions have resulted in assay interference, causing TSH to be spuriously lowered. Smaller doses of biotin found in dietary supplements also have caused assay interference. METHODS: We describe four cases in which over-the-counter (OTC) biotin caused erroneous thyroid diagnosis and clinical confusion in patients with preexisting thyroid disease. Serum TSH and thyroxine were measured by the Vitros 5600 assay. CONCLUSIONS: Although the biotin-streptavidin interaction is sensitive for detecting serum levels of TSH, it is subject to interference by exogenous biotin at levels found in OTC products. The widespread use of OTC biotin for cosmetic purposes can adversely affect the diagnosis of the entire spectrum of functional thyroid disorders. Physicians must carefully and routinely question for the use of biotin before thyroid function testing.

Radioactivity of blood samples taken from thyroidectomized thyroid carcinoma patients after therapy with (131)I.

BACKGROUND: Occasionally, blood samples may be required from thyroid cancer patients after they have been given the therapy dose of (131)I, as part of necessary medical management of comorbidities. Thus, in the days after (131)I administration, medical health professionals may be involved in the withdrawal, handling, and manipulation of radioactive blood samples. The purpose of this study was to quantify the amount of radioactivity in blood samples taken from thyroidectomized thyroid carcinoma patients after the administration of therapeutic activities of (131)I. METHODS: For dosimetry purposes, serial blood sampling is performed on thyroidectomized thyroid carcinoma patients prior to therapy with (131)I. The quantities of radioactive material present in these blood samples were expressed as a percentage of the administered activity and then extrapolated to the high levels of (131)I used in therapy for 377 patients in this study. The corresponding radiation exposure rate from the blood samples was then calculated to determine what radiation protection methods were required for staff handling these samples. RESULTS: The average amount of radioactivity in a 1 mL blood sample at 1 hour postadministration of 5.5 GBq (150 mCi) of (131)I was 0.2 ± 0.15 MBq (5.4 ± 4.0 µCi). This corresponds to an exposure rate of 1.23 µSv/h (0.123 mrem/h) at 10 cm from the sample. For samples obtained beyond 24 hours after a therapeutic administration of 5.55 GBq (150 mCi), the exposure levels are approximately equal to background radiation. CONCLUSION: The data in this study indicate that the radiation exposure from blood samples withdrawn from thyroidectomized thyroid cancer patients is low. However, to ensure that staff members are exposed to minimal levels of radiation, it is imperative that staff members who are involved in withdrawing, handling, or manipulating radioactive blood samples adhere to the recommended radiation safety practices.