How to Write an Order for Repeat Thyroid Function Tests to Evaluate Assay Interference
When you suspect assay interference in thyroid function tests, order the repeat labs on a different analyzer platform that uses a different immunoassay methodology, and ensure the patient has discontinued biotin supplementation for at least 72 hours before testing.
Key Steps for Ordering Repeat Labs
1. Specify a Different Analyzer Platform
The most critical element of your order is requesting analysis on a different immunoassay platform 1. Write your order to explicitly state:
- "Repeat TSH, free T4 [and free T3 if indicated] using different analyzer/methodology than [specify original platform if known]"
- Common platforms use different technologies: electrochemiluminescence (Roche Cobas), chemiluminescent microparticle immunoassay (Abbott Architect/Alinity), or other competitive immunoassays (Beckman, Siemens)
Why this matters: One-step immunoassays are particularly susceptible to interference from thyroid hormone autoantibodies, while two-step assays may show different results 2. Method comparison is the cornerstone of detecting interference 3.
2. Address Biotin Interference
Include in your order: "Patient to discontinue all biotin-containing supplements for at least 72 hours prior to blood draw" 1.
Biotin interference is increasingly common and can create a false biochemical profile mimicking Graves' disease (low TSH with very high thyroid hormones) in clinically euthyroid patients 1. If biotin cannot be stopped, specify: "Use analyzer not utilizing streptavidin-biotin bonding" 1.
3. Timing of Repeat Testing
The appropriate interval depends on clinical context 4, 5:
- Urgent situations (atrial fibrillation, cardiac disease, suspected thyrotoxicosis): Repeat within 2-4 weeks 5, 6
- Non-urgent discordant results: Repeat within 4-12 weeks 4, 5
- Stable patients with mild abnormalities: Can wait 3 months 5
4. What to Order
Based on the initial abnormality 5:
- If TSH < 0.1 mIU/L: Order TSH, free T4, and free T3 (or total T3)
- If TSH 0.1-0.45 mIU/L: Order TSH and free T4
- If elevated TSH with incongruously high free T4: Order TSH and free T4 on different platform 2
5. Additional Testing to Consider Ordering Simultaneously
When interference is strongly suspected 3:
- Polyethylene glycol (PEG) precipitation - helps identify high-molecular-weight interferents like thyroid hormone autoantibodies or macro-TSH
- Dilution studies - if available through your laboratory
- Heterophile antibody blocking reagents - for suspected heterophile antibody interference
Common Pitfalls to Avoid
Don't simply repeat the test on the same analyzer - this will reproduce the same interference 3, 7. The interference is method-specific.
Don't forget to document the clinical-biochemical discordance in your order. Note why you suspect interference (e.g., "clinically euthyroid with biochemical hyperthyroidism" or "symptoms inconsistent with lab values").
Don't order during acute illness unless diagnosis would alter immediate management 4, 5. Thyroid function tests are frequently disturbed non-specifically during acute non-thyroid illness, especially in hospitalized patients 4, 5.
Sample Order Language
"Repeat thyroid function tests (TSH, free T4, free T3) using different immunoassay platform than previous [specify if known: e.g., 'not Roche Cobas']. Patient to discontinue biotin x 72 hours prior to draw. Reason: suspected assay interference - biochemical results discordant with clinical presentation. Consider PEG precipitation if available to evaluate for thyroid hormone autoantibodies."
Clinical Context Matters
The evidence shows that >50% of documented thyroid assay interferences lead to misdiagnosis and inappropriate management, including unnecessary treatment with adverse effects, inappropriate medication changes, or unnecessary imaging 3. Strong interaction between clinician and laboratory is essential - call your laboratory to discuss suspected interference and available alternative methodologies 3, 8.