Biotin (vitamin B7) is a water-soluble vitamin and well known as a coenzyme for multiple bicarbonate-dependent carboxylases essential for human metabolism.¹⁾ The Food and Nutrition Board of the National Research Council recommends 30 μg/day biotin as adequate intakes for adults.²⁾ Recent studies have shown the benefit of biotin for medical conditions, such as hair and nail problems, diabetes mellitus,³⁾ peripheral neuropathy,⁴⁾ and multiple sclerosis;⁵⁾ therefore, over-the-counter biotin supplements have become popular.

Because of the high affinity to streptavidin, biotin has a crucial role in immunoassays.⁶⁾ Excessive biotin intake can affect laboratory test results, and it can mislead physicians resulting in inappropriate treatments for patients.⁷⁾

There have been no reports in South Korea about biotin-induced interference in thyroid function tests (TFTs). We report three patients with papillary thyroid cancer (PTC) who showed false thyrotoxicosis on follow-up studies with a literature review.

The streptavidin-biotin complex has been used in a variety of immunoassays. The analyte concentration and the signal are inversely proportional in competitive assays (e.g., free T4 and total T3). If the biotin concentration is supraphysiological, it competes with the biotinylated analytes and the assay shows a low signal. As a result, free T4 and total T3 will be higher than the true value. In contrast, the signal increases in proportion to the analyte concentration in uncompetitive sandwich assays (e.g., TSH). Excess biotin will compete with the analyte complex and show results that are lower than the true value.^8,9) As a consequence, the TSH level will be falsely low.

The patients’ drug history must be determined to avoid assay interference, and any suspicious supplements containing biotin should be identified. Although we were not able to identify biotin contents in the health supplements in Case 2, we assumed that fruit extracts in the supplements or biotin-producing microbes in the probiotics product might contribute to the elevation of serum biotin level.¹⁰⁾ The relationship between the probiotics and the biotin-utilizing immunoassays needs to be further studied. We considered that the possibility of thyroiditis would be unlikely because her TFT recovered in only 1 week after withdrawing the supplements, her anti-thyroglobulin antibody levels were within normal range during the follow up visits, and T3-dominant thyrotoxicosis did not fit with the typical findings of thyroiditis.¹¹⁾ In Case 3, later, we were informed that royal jelly is the source for biotin. A study has shown that royal jelly contains 84.3 nmol/g biotin.¹²⁾ Thus, any supplements that may contain biotin must be discontinued before retesting. The time needed to restrain from using the supplements increases with the biotin dose because biotin level is positively correlated with the strength of interference.¹³⁾ The American Association for Clinical Chemistry guidelines for biotin interference on laboratory tests recommends at least an 8 hour withdrawal period for patients who take 5-10 mg biotin and 72 hours for higher doses of biotin (>100 mg/day) before blood tests.¹⁴⁾ Secondly, if it is impossible to withhold the supplements, non-biotinylated assays are another option. Li et al.¹⁴⁾ compared multiple immunoassays with different assay systems related to biotin intake. According to that study, biotinylated assays, such as the Roche Cobas assay, show significantly different total T3, free T4, and free T3 after ingesting biotin.¹⁴⁾ As clinical laboratories in Chonnam University Hwasun Hospital have used the Roche Cobas assay system for thyroid immunoassays, the test results were concordant with previous case reports that used biotinylated assays (Table 2).^8,15-19) Piketty et al.¹³⁾ suggested effective neutralization methods to suppress biotin interference, which needs further studies to be widely used.

It is difficult to be aware of biotin interference when a PTC patient who is on levothyroxine suppressive therapy consumes a biotin supplement because we commonly suppress TSH levels to prevent the recurrence of thyroid cancer. Excess biotin intake misleads the physician to decide to lower the dose of levothyroxine, which may be suboptimal for the patient.

This report is the first case series of biotin-induced thyroid immunoassay interference in South Korea. Biotin interference should be considered when TFT results are discordant with clinical findings because biotin use is widespread, and some health supplements contain concealed biotin. If interference by biotin is suspected, repeating the TFT after withholding the biotin supplement for 1 week is a reasonable approach.