What is the role of third-generation thyroid-stimulating hormone (TSH) assays in the diagnosis of hypothyroidism, and how might biotin interfere with screening?

Updated: Mar 03, 2021
  • Author: Philip R Orlander, MD, FACP; Chief Editor: George T Griffing, MD  more...
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Third-generation thyroid-stimulating hormone (TSH) assays are generally the most sensitive screening tool for primary hypothyroidism. [1] If TSH levels are above the reference range, the next step is to measure free thyroxine (T4) or the free thyroxine index (FTI), which serves as a surrogate of the free hormone level. Routine measurement of triiodothyronine (T3) is not recommended.

Biotin, a popular health supplement, may interfere with immunoassays of many hormones, resulting in values that are falsely elevated or suppressed, including for thyroid levels. To avoid misleading test results, the American Thyroid Association recommends cessation of biotin consumption at least 2 days prior to thyroid testing. [2]

Results in patients with hypothyroidism are as follows:

  • Elevated TSH with decreased T4 or FTI

  • Elevated TSH (usually 4.5-10.0 mIU/L) with normal free T4 or FTI is considered mild, or subclinical, hypothyroidism [3]

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