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The thyroid axis and immunoassay pitfalls

16 min

Skills you'll build3 skills
  • Locate an endocrine defect in the gland or pituitary from target and regulating hormones
  • Classify a TSH and free T₄ pattern after checking dose timing, binding, and interference
  • Test a discordant hormone immunoassay by dilution, blocking reagent, or another platform

At the bench: Assess specimen interference before reporting a chemistry result

Try first

Try first

A man had pituitary surgery last year. His free T4 is 0.6 ng/dL (reference interval 0.9 to 1.7 ng/dL) and his TSH is 1.2 mIU/L (reference interval 0.30 to 4.20 mIU/L). He takes no biotin and no thyroid medication, and a repeat specimen a week later gives the same results. How do you interpret the panel?

Keep going to see the reasoning.

Keep going to see the reasoning.

Keep going to see the reasoning.

Right. Keep going to follow the reasoning.

Keep going to see the reasoning.

Get the idea

Use the explanation to check your first decision.

Interpreting TSH with free T4

Thyroxine (T4) and triiodothyronine (T3) suppress thyrotropin-releasing hormone and thyroid-stimulating hormone (TSH). In an intact axis, TSH tracks the free hormone the pituitary senses, so a thyroid panel pairs TSH with free T4.1 Together they localize the defect.

TSHFree T4Interpretation
HighLowPrimary thyroid failure
Low or inappropriately normalLowCentral disease, pituitary or hypothalamic
LowHighThyrotoxicosis
Normal or highHighCheck timing, binding and interference first

Dose timing and protein binding

A pattern that fits no recognized category, or that differs from the patient's earlier results, is checked before it is classified.2

  • Dose timing. Free T4 rises about 15% for several hours after a levothyroxine dose. Record the time of the last dose with the draw time.3
  • Binding. Estrogen and pregnancy raise thyroxine-binding globulin. Total T4 rises and free T4 changes little.4
  • Repeat. An isolated abnormal result is repeated before it is classified.2

Testing for immunoassay interference

Three interferences affect every run of the same specimen in the same assay. A repeat on the same analyzer reproduces them, so agreement between runs does not confirm the result.5

  • Biotin. High-dose biotin competes for streptavidin. Sandwich assays such as TSH read falsely low, and competitive assays such as many free T4 methods read falsely high. A new specimen drawn after the manufacturer's washout interval confirms or excludes it. That interval can reach 72 hours or more with high doses.5,6
  • Heterophile antibodies. They usually bridge the two antibodies of a sandwich assay and raise the result. Test with a heterophile-neutralizing reagent or on another platform. A dilution series can look linear despite this interference.5
  • Hook effect. Extreme antigen excess saturates both antibodies of a sandwich assay separately, and the result reads falsely low. Serial dilution detects it. The reportable value is where two successive dilutions agree after correction.5

Keep a result with a suspected interference pending release until the test of that mechanism is done.

Follow the bench demonstration

Sources for this step
References
  1. Van Uytfanghe K, Ehrenkranz J, Halsall D, et al. Thyroid stimulating hormone and thyroid hormones (triiodothyronine and thyroxine): an American Thyroid Association-commissioned review of current clinical and laboratory status. Thyroid. 2023;33(9):1013-1028. doi:10.1089/thy.2023.0169
  2. Ross DS, Burch HB, Cooper DS, et al. 2016 American Thyroid Association guidelines for diagnosis and management of hyperthyroidism and other causes of thyrotoxicosis. Thyroid. 2016;26(10):1343-1421. doi:10.1089/thy.2016.0229
  3. Jonklaas J, Bianco AC, Bauer AJ, et al. Guidelines for the treatment of hypothyroidism: prepared by the American Thyroid Association task force on thyroid hormone replacement. Thyroid. 2014;24(12):1670-1751. doi:10.1089/thy.2014.0028
  4. Rifai N, Chiu RWK, Young I, Burnham CAD, Wittwer CT, eds. Tietz Textbook of Laboratory Medicine. 7th ed. Elsevier; 2023.
  5. Ghazal K, Brabant S, Prie D, Piketty ML. Hormone immunoassay interference: a 2021 update. Ann Lab Med. 2022;42(1):3-23. doi:10.3343/alm.2022.42.1.3
  6. US Food and Drug Administration. Testing for Biotin Interference in In Vitro Diagnostic Devices: Guidance for Industry. US Food and Drug Administration; October 2020. Accessed September 27, 2026. https://www.fda.gov/media/127915/download

Watch one

Read each decision and its explanation.

A woman with primary hypothyroidism has her follow-up panel drawn at 10:30. The requisition records her levothyroxine dose at 08:00 and no biotin or heparin. Her previous panel was drawn at 07:30, before that day's dose.

How do you interpret this panel?

TestResultPreviousReference intervalFlag
TSH2.2 mIU/L8 weeks ago: 2.3 mIU/L2.3 mIU/L8 weeks ago0.30–4.20 mIU/L0.30–4.20
Free T41.8 ng/dL High8 weeks ago: 1.6 ng/dL1.6 ng/dL8 weeks ago0.9–1.7 ng/dL0.9–1.7High

Specimen: H 8, L 15, I 1. Serum, last levothyroxine dose 08:00, drawn 10:30

  1. Review TSH and free T4 together. A high free T4 with a TSH within its interval does not fit an intact axis.

    The pituitary responds to a high free T4 by lowering TSH, so the two are interpreted together.

  2. Compare with the previous panel. TSH went from 2.3 to 2.2 mIU/L, and only free T4 changed.

    TSH is the steady-state marker in primary disease, and it changed very little.

  3. Check the dose timing. This specimen was drawn 2.5 hours after the 08:00 dose, and the previous specimen was drawn before the dose.

    Free T4 peaks for several hours after an oral dose.

  4. Calculate the change: (1.8 − 1.6) ÷ 1.6 × 100 = 12.5%, which fits the post-dose rise.

    A post-dose peak of about 15% explains a change of this size.

  5. Check the requisition for binding and interference. No biotin or heparin is recorded.

    Biotin and heparin are the other common causes of a high free T4 with a normal TSH.

The free T4 of 1.8 ng/dL is a post-dose peak. The panel shows no new thyroid pattern. Report it with the dose and draw times in a comment.

Practice this skill

Your turn

Problem 1 of 3 · Hints available

Verified results show a low free T₄ and a markedly increased TSH. Which axis pattern does this support?

Correct. With the axis intact, the pituitary raises TSH when thyroid hormone output falls, so a low free T4 with a high TSH places the failure in the thyroid.

Incorrect. Central disease gives a low, normal, or only mildly raised TSH that is inappropriate for the low free T4. A markedly increased TSH shows the pituitary responding.

Incorrect. A TSH-secreting adenoma drives free T4 up, so its high TSH comes with a high free T4.

Hint
  1. Consider how the pituitary responds when free T4 falls.
  2. A pituitary that responds raises the TSH.

Review Feedback and axis localization

Problem 2 of 3 · Hints available

A thyroid panel shows a high free T₄ and a normal TSH, which conflicts with the patient's previous results. The requisition notes a high-dose biotin supplement. Which investigation should come before release?

Incorrect. The same specimen still contains the biotin, so a repeat on the same analyzer reproduces the interference, and agreement between runs does not rule it out.

Correct. Excess biotin competes for streptavidin, so competitive free T4 methods read falsely high and the sandwich TSH can read falsely low. A specimen drawn after the manufacturer's washout interval, which can reach 72 hours or more with high doses, gives a result free of biotin interference.

Incorrect. Serial dilution detects antigen excess in a sandwich assay. Biotin interference is checked with a specimen drawn after the washout interval.

Hint
  1. Consider how biotin affects every run of this specimen.
  2. The interference lasts as long as the biotin stays in the patient's blood.

Review Assay principles and failure modes

Problem 3 of 3 · On your own

A man who feels well has a routine panel. His TSH is 26 mIU/L and his free T4 is 1.4 ng/dL. Six weeks ago his TSH was 1.9 mIU/L, and he takes no thyroid medication and no biotin. A repeat of the same specimen on the same analyzer gives 25 mIU/L. What do you do before release?

Heterophile antibodies act on every run of this specimen in this assay, so the repeat reproduces them. Agreement between two runs cannot show whether the TSH is real.

Took agreement on a repeat immunoassay as confirmation

Biotin, heterophile antibodies, and antigen excess act on every run of the same specimen and assay, so a repeat reproduces the error. Treating agreement as confirmation releases an interfered result. Dilution, a blocking reagent, a specimen drawn after biotin washout, or another platform tests the suspected mechanism.

He takes no biotin, and biotin causes a falsely low sandwich TSH. This TSH is high.

A sudden high TSH in a well man, with an unchanged free T4, points to heterophile antibodies bridging the sandwich. A neutralizing reagent or a different platform tests that mechanism, and a large fall in the TSH shows the interference.

Review Assay principles and failure modes

Use it

Read the case evidence, then make each decision. The step is complete without it.

  • A man has a large pituitary mass on imaging, and the requisition asks for prolactin.
  • He takes no biotin.
  • The sandwich prolactin method reads up to 470 ng/mL, and its adult male reference interval is 4.0 to 15.2 ng/mL.
  • The first result is 85 ng/mL. Your procedure calls for a dilution study when a large mass comes with a modest prolactin.
  • Dilution-corrected results: 1:10 gives 2,400 ng/mL, 1:100 gives 5,200 ng/mL, and 1:1,000 gives 5,100 ng/mL.
Decision 1 of 2

What explains the first result of 85 ng/mL?

Macroprolactin reacts strongly in prolactin assays and raises the result. It cannot explain a result that climbs as the specimen is diluted.

So much prolactin saturates both antibodies separately that few sandwiches form, and the undiluted result is falsely low. Each dilution reduces the excess, and the corrected result rises.

Biotin can lower a sandwich result, but he takes none. A biotin effect would not rise with dilution.

Heterophile antibodies usually bridge a sandwich and raise the result. A dilution series can look linear with them, and this one rises.

Review Assay principles and failure modes

Decision 2 of 2

What do you release?

A result within the measuring range can be falsely low from the hook effect. The dilutions show the true concentration is far higher, and 85 ng/mL would be reported as a modest rise.

The corrected result was still rising between 1:10 and 1:100, so antigen excess was still lowering the 1:10 result.

Two successive dilutions agree within 2% after correction, so the antigen excess is relieved. Release the agreed value with a comment that the undiluted result was affected by the hook effect.

An alternate method is the next step when the corrected results of a series never agree. Here two successive dilutions agree, and the value is reportable.

Review Assay principles and failure modes

The corrected result rose with dilution:

  • 85 ng/mL undiluted
  • 2,400 ng/mL at 1:10
  • 5,200 and 5,100 ng/mL at 1:100 and 1:1,000

A result that rises as the specimen is diluted indicates antigen excess in a sandwich assay. The two agreeing dilutions give the value to report.

For reference

Sources checked