Endocrine Abstracts

Biotin is used in many hormone immunoassays as an immobilizing system as streptavidin binds biotin with high affinity. Recently, cases with spurious results from biotin interference have been increasing due to widespread use of vitamin supplements containing biotin. We report two cases of erroneously high total T3 levels caused by biotin interference.

Case 1: A 88-year-old female who had total thyroidectomy for papillary thyroid cancer 10 years ago and had been taking levothyroxine 125 µg/day underwent routine follow-up thyroid function tests (TFT). Surprisingly, her serum total T3 was >651 ng/dl (60–160) and TSH 0.65 µIU/ml (0.4–4.8). Hormones were measured with a chemiluminescent immunoassay (Cobas: Roche Diagnostics, West Sussex, UK). She had no symptoms and signs of thyrotoxicosis. Subsequent detailed history revealed the culprit. Vitamin supplement (Adult multivitamin Gummies, Kirkland Signature, US) she had been taking since several months ago contained 300 µg biotin per tablet. Her serum total T3 level normalized to 110 ng/dl 1 week after stopping the vitamin.

Case 2: A 43-year-old female who had left lobectomy for papillary thyroid cancer 9 years ago and no levothyroxine replacement underwent routine follow-up TFT. Although she had no signs of thyrotoxicosis, her serum total T3 was 633 ng/dl, free T4 2.42 ng/dl, and TSH 0.786 µIU/ml. She had been taking several complex health supplements including vitamins, probiotics and diet products. Biotin was not identifiable in the label on the products. Repeated TFT results was normalized 1 week after stopping all supplements.

Biotin interference should be considered when TFT results are not compatible with clinical findings, because biotin use is widespread, and some health supplements contain concealed biotin. If biotin interference is suspected, repeating the TFT after stopping biotin supplement for one week can be a reasonable approach.