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Thyroid Uptake

SI UNITS (recommended)

CONVENTIONAL UNITS

(Radioiodine Uptake Test - Measures Thyroid Trapping of Iodine)

Synonyms

  • Radioiodine uptake
  • RAIU
  • Thyroid iodine uptake
  • I-123 uptake
  • I-131 uptake
  • Thyroidal iodine trapping test
  • 24-hour uptake

Units of Measurement

  • Percentage (%)

(This test is always expressed as a % of administered radioactive iodine dose taken up by the thyroid.)

Description

Thyroid Uptake (%) measures the percentage of a given dose of radioactive iodine (I-123 or I-131) that is taken up by the thyroid gland at a specific time (commonly 2–6 hours and 24 hours).

It evaluates:

  • Thyroid iodine-trapping function
  • Hormone synthesis activity

Primarily used to diagnose:

  • Hyperthyroidism causes
  • Thyroiditis vs. Graves’ disease
  • Functional autonomy (toxic nodules)
  • Preparation for radioiodine therapy

Physiological Basis

Thyroid uptake reflects:

  1. Sodium–iodide symporter (NIS) function
  2. Iodine trapping in follicular cells
  3. Organification (iodine incorporation into thyroglobulin)

High uptake = thyroid is actively making hormone.
Low uptake = thyroid is “quiet” or damaged.

Reference Intervals

(SNMMI + ATA + Mayo + Tietz)

Normal Uptake

TimeNormal Range
2–6 hours5–15%
24 hours10–30%

(Ranges vary by geography due to dietary iodine intake.)

Interpretation

HIGH UPTAKE (%)

Indicates overactive thyroid hormone production.

Causes

1. Graves’ Disease (most common)

  • Diffusely high uptake
  • Homogeneous distribution

2. Toxic Multinodular Goiter

  • Patchy areas of high uptake

3. Toxic Adenoma

  • One hyperfunctioning “hot” nodule
  • Suppressed uptake in surrounding tissue

4. Iodine Deficiency

  • Thyroid compensates → high uptake

5. Rebound after antithyroid drugs

Temporarily increased uptake.

LOW UPTAKE (%)

Indicates reduced iodine trapping or thyroid destruction.

Causes

1. Thyroiditis (destructive)

  • Subacute thyroiditis (De Quervain)
  • Painless / silent thyroiditis
  • Postpartum thyroiditis
  • Radiation thyroiditis

2. High Iodine Load (Wolff–Chaikoff effect)

  • Recent contrast CT
  • Amiodarone therapy
  • Iodine-containing supplements

3. Exogenous Thyroid Hormone Intake

(Factitious thyrotoxicosis)

4. Hypothyroidism

Due to:

  • Hashimoto thyroiditis
  • Post-surgery
  • Post-radioiodine therapy

5. Recent Universal Salt Iodization exposure

High iodine intake → low uptake.

Patterns of Uptake & Clinical Meaning

1. High Diffuse Uptake

Graves’ disease

2. High Patchy Uptake

Toxic multinodular goiter

3. Single “Hot” Nodule

Toxic adenoma

4. Near-zero Uptake

Thyroiditis or exogenous thyroid hormone use

5. High Uptake with Low TSH but normal T3/T4

Early Graves’ / latent hyperthyroidism.

Diagnostic Uses

1. Hyperthyroidism Workup

  • Graves’ vs. thyroiditis
  • Functioning nodule assessment

2. Pre-Radioiodine Therapy Planning

High uptake → better RAI response.
Low uptake → consider alternative therapy.

3. Evaluation of Iodine Metabolism Disorders

4. Post-radioiodine monitoring

Residual uptake indicates active tissue.

Analytical Notes

  • Test uses I-123 (preferred) or I-131.
  • Avoid recent iodine-containing contrast for 4–6 weeks.
  • Hold antithyroid drugs:
    • Methimazole: 3–5 days before test
    • PTU: 5–7 days before
  • Avoid kelp/iodine supplements.
  • Nuclear medicine camera measures neck counts vs. reference standard.

Clinical Pearls

  • If thyroiditis, uptake is very low, but ESR/CRP elevated and thyroglobulin high.
  • In factitious thyrotoxicosis, uptake is low but thyroglobulin very low (telltale clue).
  • Amiodarone patients may have low uptake even with symptoms of thyrotoxicosis.
  • In iodine-deficient regions, “normal” uptake may be higher.
  • RAI uptake must be interpreted alongside TSH, Free T4, Total T3, and clinical signs.

Interesting Fact

I-123 is ideal for diagnostic uptake because it emits gamma rays only, has a short half-life (~13 hours), and gives much lower radiation exposure compared to I-131.

References

  1. Tietz Clinical Chemistry & Molecular Diagnostics, 8th Edition - Thyroid Function Tests
  2. SNMMI Procedure Standards - Radioiodine Uptake & Imaging
  3. American Thyroid Association (ATA) Hyperthyroidism Guidelines
  4. Mayo Clinic Laboratories - Radioiodine Uptake
  5. ARUP Consult - Thyroid Function & Nuclear Medicine Testing
  6. NCCN - Thyroid Cancer/RAI Testing Principles

Last updated: January 27, 2026

Reviewed by : Medical Review Board

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