Thyroid tests are usually reliable, but in a small share of cases something in the blood sample “confuses” the test itself- called assay interference (“assay” = lab test) — producing a result that doesn't reflect the body's true hormone level.

What is “interference,” and how common is it?

TSH (thyroid-stimulating hormone, the pituitary's signal to the thyroid) and free T4 (fT4, the active, unbound form of thyroid hormone) are measured with immunoassays- tests that use antibodies to detect the hormone. Interference occurs when something else in the sample, usually an antibody the patient's own body has made, throws off that detection, producing a falsely high or low result. One large lab review found this in roughly 0.5% of patients- uncommon per test, but frequent enough across a health system (an estimated 50,000 misleading UK results per year) that clinicians should stay alert to it.

Interference Affecting TSH

• Antibodies against the test itself (“anti-reagent”): Lab tests often use animal-derived antibodies; some people make their own antibodies against these (e.g., animal handlers, past medical treatments using animal products, or due to normal immune system variations and autoimmune conditions), blocking the signal (falsely low) or creating a false one (falsely high). Naturally occurring “heterophile” antibodies cause the same effect more commonly.
• Antibodies against TSH itself (“macro-TSH”): These form large, inactive antibody-TSH complexes that many tests still detect, producing a falsely high result despite normal thyroid function.

Interference Affecting Free T4

Less than 0.1% of T4 circulates “free” (unbound); isolating that sliver accurately is technically difficult, which makes fT4 more prone to interference:

• Fatty-acid displacement: Heparin (a blood thinner) releases fatty acids that knock T4 off its carrier protein, inflating the test-tube reading without reflecting true in-body levels.
• Familial dysalbuminemic hyperthyroxinemia (FDH): A harmless inherited (“familial”) condition where albumin binds T4 too tightly, falsely raising total and sometimes free T4 despite normal thyroid function.
• Anti-T4 or anti-reagent antibodies: Work the same way as the TSH antibody interference described above.

A Common Cause Worth Knowing: Biotin

Biotin (vitamin B7, found in high doses in many hair, skin, and nail supplements) is one of the most common real-world causes of thyroid test interference, though it works differently from the causes above. Many thyroid immunoassays use a biotin-based chemical step to hold the test together (a “streptavidin-biotin” link). If someone has taken a high-dose biotin supplement recently, the extra biotin floods this step and throws off the reading- typically producing a falsely low TSH alongside a falsely high free T4 and free T3- a pattern that can look just like an overactive thyroid even when the thyroid is completely normal. This is dose-dependent: typical multivitamin amounts aren't a concern, but the 5,000–10,000 mcg doses common in hair/skin/nail products can cause it. The fix is simple- stop biotin supplements for about 48–72 hours before testing, and mention any biotin use to your provider or the lab if a result looks unexpectedly thyrotoxic.

Bottom Line

Thyroid results should always be read alongside symptoms and history, not in isolation. Interference can't always be resolved from the original sample alone- sometimes a retest is necessary. Patients and providers shouldn't panic over one unexpected result and may request a retest when a result doesn't fit the clinical picture.