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Thyroid scintigraphy and uptake measurement
In thyrotoxicosis with a suppressed TSH, thyroid scintigraphy answers one question — is the gland making hormone (high uptake: Graves disease, toxic nodule, toxic multinodular goitre) or only releasing stored or external hormone (low uptake: thyroiditis, iodine excess, exogenous hormone)?
Why it matters in the exam
Exam questions are built on a patient with a low TSH and a high free T4 and a scan image that asks for the diagnosis: diffuse, increased uptake with low background fits Graves disease; one hot nodule suppressing the rest of the gland fits a toxic adenoma; several hot and cold nodules side by side fit a toxic multinodular goitre; and a barely visible gland with high background fits subacute thyroiditis, silent or postpartum thyroiditis, iodine load from contrast medium or covert levothyroxine intake.
The tracer is chosen by the question. Tc-99m pertechnetate is trapped by the gland through the sodium-iodide symporter (NIS) but is not organified; it is cheap, available, imaged about 20 minutes after injection and suits most cases. I-123 is trapped and organified, so it is preferred when an accurate uptake value at 4 and 24 hours is needed, when a lingual or retrosternal gland is suspected, and when an organification defect is suspected (perchlorate discharge test). A nodule that is hot on pertechnetate and cold on I-123 is called discordant and is a reason for cytological work-up.
Normal 24-hour I-123 uptake is roughly 10%–30% (the range depends on the population's iodine intake). An iodine load (injected contrast medium, amiodarone, laxatives or seaweed supplements) lowers uptake for weeks to months, so it must be asked about before the study. Antithyroid drugs are stopped several days before the study. Scintigraphy is contraindicated in pregnancy, and breastfeeding is interrupted for a set time that depends on the tracer. The study also sets the I-131 activity for treating hyperthyroidism and finds ectopic thyroid tissue in children with congenital hypothyroidism.
Key points
- Thyroid scintigraphy separates overproduction (high uptake) from release or an external source (low uptake).
- Graves — diffuse, high uptake; toxic adenoma — hot nodule with gland suppression.
- Pertechnetate is trapped only; I-123 is trapped and organified.
- An iodine load lowers uptake for weeks to months.
- Normal 24-hour uptake is roughly 10%–30%.
Summary table
| Diagnosis | Scan pattern | 24-hour uptake | Other clue |
|---|---|---|---|
| Graves disease | Diffuse, homogeneous uptake, low background, sometimes a pyramidal lobe | High | TRAb antibodies, ophthalmopathy |
| Toxic adenoma | One hot nodule, rest of the gland suppressed | Normal to high | Palpable nodule |
| Toxic multinodular goitre | Hot and cold nodules side by side | Normal to slightly high | Older adults, iodine-deficient areas |
| Thyroiditis (subacute, silent, postpartum) | Gland barely visible | Very low | Pain and high ESR in subacute |
| Iodine excess or exogenous hormone | Gland not visible | Very low | Low thyroglobulin with exogenous hormone |
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Sources
- SNMMI Procedure Standard for Thyroid Scintigraphy and Uptake Measurements, 2021 — Radiopharmaceuticals, indications and interpretation
- EANM Practice Guideline for Thyroid Scintigraphy, 2019 — Indications and image interpretation
- American Thyroid Association Guidelines for Diagnosis and Management of Hyperthyroidism and Other Causes of Thyrotoxicosis, 2016 — Determining the aetiology of thyrotoxicosis
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.