12.4 Thyroid Uptake Measurement and Radioiodine Survey Technique
Key Takeaways
- Percent radioactive iodine uptake equals (neck counts minus thigh background) divided by (decay-corrected standard counts minus room background), multiplied by 100—every term must be collected with matched geometry and count time.
- The standard is a capsule identical to the administered dose counted in a neck phantom at the same probe distance, and it must be decay-corrected to the time of the patient count before the ratio is formed.
- Typical adult 24-hour uptake reference ranges run roughly 10–30 percent, but each laboratory establishes its own values; high uptake with thyrotoxicosis suggests Graves disease or a toxic nodule, while low uptake suggests thyroiditis or exogenous hormone.
- Recent iodinated contrast, amiodarone, kelp or iodine supplements, and thyroid medications suppress measured uptake—amiodarone can interfere for many months, so always document the exposure date.
- Whole-body radioiodine surveys use I-123 or low-activity diagnostic I-131 after TSH stimulation by hormone withdrawal or recombinant human TSH plus a low-iodine diet; larger diagnostic I-131 activities risk stunning subsequent therapy uptake.
12.4 Thyroid Uptake Measurement and Radioiodine Survey Technique
Quick Answer: %RAIU = [(neck − thigh) ÷ (decay-corrected standard − room background)] × 100. Count the standard in a neck phantom at the same distance as the patient, use the same count time, and decay-correct the standard to the counting moment. Typical adult 24-hour uptake is roughly 10–30% (laboratory-specific). Iodine load suppresses uptake—contrast for weeks, amiodarone for months.
Section 12.2 covered thyroid imaging clinically and Section 9.1 covered the uptake probe as hardware. This section is the measurement itself: geometry, counts, arithmetic, and the interferences that make the number meaningless.
The Instrument and Its Geometry
A thyroid uptake system is a 2-inch × 2-inch NaI(Tl) scintillation detector with a flat-field (open-bore) collimator, a photomultiplier tube, and a pulse-height analyzer set on the appropriate photopeak (159 keV for I-123, 364 keV for I-131).
| Geometry rule | Why it matters |
|---|---|
| Fixed probe-to-neck distance (commonly about 25–30 cm, per the system) | Counts fall off with the inverse square of distance; a 10% distance error is a large count error |
| Same distance for the standard in a neck phantom | The ratio only cancels geometry if both terms share it |
| Consistent patient positioning — supine or seated, neck extended, chin up | Reproducibility across the 4–6 hour and 24 hour points |
| Same count time for every measurement | Counts, not count rates, go into the ratio unless you normalize |
Before any patient measurement, verify the daily constancy and energy peaking of the counting system, and run a chi-square test on the schedule the department sets (Section 9.1).
The Four Counts
| Count | What it is |
|---|---|
| Standard | A capsule from the same lot and activity as the patient dose, counted in a neck phantom at the standard distance, before the dose is given |
| Room background | The counting room with the standard and patient removed—subtracted from the standard |
| Neck (patient) | Over the thyroid at the fixed distance at each protocol time point |
| Thigh (patient background) | Over the mid-thigh at the same distance and time, representing circulating and soft-tissue activity — subtracted from the neck count |
The Calculation
%RAIU = [(neck counts − thigh counts) ÷ (standard counts − room background)] × 100
with the standard decay-corrected from its counting time to the time of the patient count.
Worked example — 24-hour I-123 uptake.
- Standard counted at administration: 420,000 counts/min; room background 300 counts/min → net standard 419,700
- I-123 half-life 13.2 h; elapsed 24 h → decay factor = (½)^(24/13.2) = (½)^1.818 ≈ 0.284
- Decay-corrected net standard at 24 h ≈ 419,700 × 0.284 ≈ 119,200 counts/min
- Patient neck at 24 h: 28,000 counts/min; thigh: 1,400 counts/min → net neck 26,600
%RAIU ≈ 26,600 ÷ 119,200 × 100 ≈ 22%
That result sits inside a typical adult 24-hour reference band. Note what happens if you forget to decay-correct: 26,600 ÷ 419,700 ≈ 6.3%, a falsely "low uptake" result that could be misread as thyroiditis. Decay correction is the single most common arithmetic failure on this study.
Reference Ranges and Interpretation
| Pattern | Teaching interpretation |
|---|---|
| High uptake + thyrotoxic labs | Graves disease, toxic multinodular goiter, toxic adenoma |
| Low uptake + thyrotoxic labs | Thyroiditis (subacute, silent, postpartum), exogenous thyroid hormone, recent iodine load |
| Low uptake + hypothyroid labs | Advanced Hashimoto disease, prior ablation or surgery |
| Elevated early (4–6 h) uptake with rapid fall by 24 h | Rapid-turnover Graves pattern; some labs add early time points for therapy dose calculation |
Reference ranges differ by laboratory and by dietary iodine intake in the served population — use your own laboratory's values, commonly around 5–15% at 4–6 hours and 10–30% at 24 hours.
Interferences That Invalidate the Measurement
| Exposure | Approximate interference window |
|---|---|
| Iodinated IV or oral CT contrast | Weeks to months |
| Amiodarone | Many months (large iodine reservoir in fat) |
| Kelp, iodine supplements, povidone-iodine antiseptic | Days to weeks |
| Levothyroxine / liothyronine | Suppresses TSH drive; held only on physician order |
| Antithyroid drugs (methimazole, PTU) | Alter organification and uptake |
| Recent radiopharmaceutical administration | Residual activity contaminates counts |
Document the date of any exposure — an item stem that mentions a CT with contrast last week and a "surprisingly low uptake" is testing exactly this.
Whole-Body Radioiodine Surveys
After thyroidectomy for differentiated thyroid cancer, whole-body imaging maps iodine-avid remnant and metastases.
| Element | Standard practice |
|---|---|
| TSH stimulation | Thyroid hormone withdrawal (endogenous TSH rise) or recombinant human TSH (Thyrogen®) injections |
| Diet | Low-iodine diet, commonly about 1–2 weeks before dosing |
| Tracer | I-123 whole-body, or diagnostic I-131 at low activity (commonly a few millicuries, protocol and regulation dependent) |
| Imaging times | I-123 often about 24 hours; diagnostic I-131 may add 48–72 hour views |
| Acquisition | Whole-body sweep plus spot views; high-energy collimator for I-131, low-energy high-resolution for I-123 |
| Post-therapy scan | Performed days after a therapy administration and is more sensitive than the diagnostic scan because of the far higher activity |
Stunning is the high-yield concept: a larger diagnostic I-131 activity can transiently reduce the uptake of the subsequent therapeutic dose. Many centers avoid it by using I-123 or minimal diagnostic I-131 activity.
Physiologic activity on a survey — salivary glands, stomach, bowel, bladder, nasopharynx, and sometimes lactating breast — must not be called metastatic. Contamination from saliva, urine, or sweat is the other classic false positive; have the patient void, wash, and change into a gown before repeat views.
Bottom line: matched geometry, four counts, decay-corrected standard, documented iodine history. Get those right and the number means something.
A 24-hour I-123 thyroid uptake is calculated. Which formula is correct?
A technologist omits decay correction of the standard when calculating a 24-hour I-123 uptake. What is the effect on the reported result?
Which patient history most strongly explains an unexpectedly low 24-hour radioiodine uptake in a clinically thyrotoxic patient?