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Short read · Endocrine

The Perchlorate Discharge Test

Endocrine · 2 min read

Trapping and organification are two separate steps. This is the test that tells them apart.

Trapping and organification are two separate steps. A gland can do the first and fail the second, and this is the only study that tells them apart.

Principle

  • Iodide enters the follicular cell on the sodium-iodide symporter.
  • Thyroid peroxidase binds it to thyroglobulin. That step is organification.
  • Perchlorate competitively blocks the symporter. Uptake stops, and free intrathyroidal iodide effluxes out.
  • Organified iodine is covalently bound and cannot leave.
  • Whatever discharges was therefore never organified. The size of the discharge measures the defect.
Pearl

Radioiodine is mandatory. 99mTc-pertechnetate is trapped but never organified, so it discharges in everyone. It images the gland; it cannot test organification.

Technique

  • Tracer. 123I, oral or intravenous. Scanning activity 7.4–14.8 MBq (0.2–0.4 mCi). A paediatric series used 3.7 MBq intravenously.
  • 131I. The original agent, and still usable for counting. Uptake-only activity is 0.185–0.37 MBq (5–10 µCi). Beta emission and an 8-day half-life give a much higher thyroid dose per unit activity, so the activity is held to microcurie levels.
  • Why 123I now. Better image quality at 159 keV, no beta, 13-hour half-life. Most discharge tests are done in neonates, where dose matters most.
  • Counting. Uptake probe over the thyroid. A gamma camera serves where no probe exists.
  • Perchlorate. Oral potassium perchlorate at 1–2 h. Count for a further 1–2 h.
  • Calculation. Fall in counts after perchlorate, as a percentage of the counts at that moment.
  • Preparation. Withhold antithyroid drugs and avoid iodine loading. Both give false positives. For re-evaluation at 3 years, stop levothyroxine 4–6 weeks first.
  • Prerequisite. The gland must trap. No uptake, no test.

Interpretation

DischargeInterpretation
Under 10%Normal
10–50%Partial defect — Pendred syndrome, DUOX2, DUOXA2
Over 50%Total defect — usually thyroid peroxidase deficiency

The threshold is 10%. Some centres use 15%.

Acquired causes

  • Carbimazole and propylthiouracil inhibit peroxidase directly.
  • Hashimoto thyroiditis gives a modest positive.
  • Iodine excess blocks organification transiently.

Clinical role and limits

Main indication

  • Congenital hypothyroidism with a eutopic gland.
  • Dysgenesis is settled by the scan alone.
  • A normal or enlarged orthotopic gland is where dyshormonogenesis must be proved.
  • In a series of 40 children, scintigraphy with a discharge test gave the diagnosis in half.

Pendred syndrome

  • Genotype first. Biallelic SLC26A4 with an enlarged vestibular aqueduct is diagnostic.
  • Test only the non-diagnostic genotype. Monoallelic carriers discharge normally.
  • The number is prognostic. Discharge tracks goitre size.
  • 10 of 12 patients discharging over 30% became hypothyroid. The hazard rose about 7% per percentage point.
Pitfall

Negative does not exclude. Positive is not always permanent. Only about 78% of genetically confirmed Pendred syndrome is positive, and a positive result in a dysgenetic or ectopic gland can normalise on retesting.

Take home
  • Perchlorate discharges iodide that was trapped but never bound. The test measures organification. Only radioiodine works.
  • Over 10% is abnormal. 10–50% is partial, over 50% total — usually peroxidase deficiency.
  • In Pendred syndrome the number is prognostic. Over 30% marks those who progress to goitre and hypothyroidism.
Sources
  1. Keller-Petrot I, Leger J, Sergent-Alaoui A, de Labriolle-Vaylet C. Congenital hypothyroidism: role of nuclear medicine. Semin Nucl Med. 2017;47(2):135-42.
  2. Soh LM, Druce M, Grossman AB, et al. Evaluation of genotype-phenotype relationships in patients referred for endocrine assessment in suspected Pendred syndrome. Eur J Endocrinol. 2015;172(2):217-26.
  3. Meller J, Zappel H, Conrad M, Roth C, Emrich D, Becker W. Diagnostic value of 123-iodine scintigraphy and perchlorate discharge test in the diagnosis of congenital hypothyroidism. Exp Clin Endocrinol Diabetes. 1997;105(Suppl 4):24-7.
  4. Li YL, Gong FY, Dang ZY, et al. Analysis of clinical characteristics of thyroid phenotype in Pendred syndrome based on multiple databases. Eur Rev Med Pharmacol Sci. 2023;27(12):5390-6.
  5. Al-Jurayyan NA, El-Desouki MI. Transient iodine organification defect in infants with ectopic thyroid glands. Clin Nucl Med. 1997;22(1):13-6.
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