NUCLEAR
NeckDemonstration dataThyroid Uptake and Scan
Thyroid scintigraphy and radioactive iodine uptake (RAIU) assess thyroid function and morphology. I-123 (or I-131 for uptake) is trapped and organified by follicular cells, giving a true measure of iodine handling; Tc-99m pertechnetate is trapped but not organified, providing a rapid perfusion/trapping image. The combination differentiates the causes of thyrotoxicosis (high uptake in Graves and toxic nodules versus low uptake in thyroiditis and exogenous causes) and characterises nodules as functioning ("hot", almost always benign) or non-functioning ("cold").
Indications
- Differentiation of the causes of thyrotoxicosis (Graves disease, toxic adenoma, toxic multinodular goitre, thyroiditis, factitious/exogenous)
- Functional characterisation of a thyroid nodule as hot or cold
- Assessment of gland size, position and ectopic or lingual thyroid tissue
- Calculation of radioactive iodine uptake before I-131 therapy dosimetry
- Evaluation of neonatal hypothyroidism (agenesis, ectopia, dyshormonogenesis)
Contraindications & Cautions
- Pregnancy is an absolute contraindication for radioiodine (I-123/I-131) as it crosses the placenta and concentrates in the fetal thyroid after ~10-12 weeks
- Breastfeeding: must be stopped for I-123/I-131 (cessation typically required for radioiodine); Tc-99m pertechnetate requires interruption for ~24 h
- Recent iodinated contrast, amiodarone or iodine-rich medication saturates the gland and lowers uptake — delay the study (weeks to months)
- Recent thyroid hormone or antithyroid medication alters uptake — withhold per protocol
- Confirm pregnancy status with a test before administering radioiodine
Patient Preparation
- Screen for recent iodinated contrast (delay 4-8 weeks), amiodarone (months), and iodine-rich supplements
- Withhold thyroid hormone (T4 ~4-6 weeks, T3 ~2 weeks) and antithyroid drugs per protocol where uptake is to be measured
- Fasting for a few hours before oral I-123 improves absorption
- Confirm pregnancy status and stop breastfeeding as required for the chosen tracer
- Explain oral capsule (iodine) versus IV injection (pertechnetate) and the timing of imaging/uptake measurement
Technique & Parameters
- Radioactive iodine uptake: administer I-123 sodium iodide 7.4-14.8 MBq orally and measure neck counts with a probe at 4-6 h and 24 h against a standard
- Iodine imaging: pinhole planar images of the neck at ~4-24 h with I-123
- Pertechnetate imaging: Tc-99m pertechnetate 74-370 MBq IV with pinhole imaging at 20-30 minutes
- Include markers (sternal notch, palpable nodule) for anatomical correlation
- Acquire anterior and oblique pinhole views to separate overlapping lobes and nodules
Systematic Review
- Calculate the RAIU percentage and compare with the normal range
- Assess gland size, position, symmetry and the presence of ectopic tissue
- Determine whether uptake is diffuse or focal/nodular
- Correlate palpable or sonographic nodules with functional (hot/cold) status
- Integrate uptake level with thyroid function tests to classify the cause of thyrotoxicosis
Key Findings & Significance
- Normal 24-hour RAIU is approximately 10-30% (laboratory-dependent)
- Diffusely increased uptake with an enlarged gland and high RAIU indicates Graves disease
- A single focus of increased uptake with suppression of the remaining gland indicates a toxic (autonomous) adenoma
- Low or absent RAIU in a thyrotoxic patient indicates thyroiditis, exogenous hormone or iodine excess
- A "cold" (non-functioning) nodule warrants ultrasound and fine-needle aspiration as ~5-15% are malignant; hot nodules are almost always benign
Differential Considerations
- High uptake thyrotoxicosis: Graves disease, toxic multinodular goitre, toxic adenoma
- Low uptake thyrotoxicosis: subacute (de Quervain) thyroiditis, silent/postpartum thyroiditis, factitious/exogenous thyroxine, iodine-induced (amiodarone type II), struma ovarii (uptake in pelvis)
- Cold nodule: colloid nodule, cyst, adenoma, carcinoma, focal thyroiditis, haemorrhage
- Diffusely decreased uptake: Hashimoto (late), iodine load, exogenous thyroid hormone
- Absent neck uptake in a neonate: agenesis versus ectopic (lingual) functioning tissue versus dyshormonogenesis
Pearls & Pitfalls
- I-123 gives a truer functional map than Tc-99m pertechnetate because iodine is organified whereas pertechnetate is only trapped
- A small proportion of nodules are discordant — functioning on pertechnetate but cold on radioiodine — so characterise suspicious nodules with iodine
- Recent iodinated CT contrast can suppress uptake for weeks and mimic thyroiditis — always take an iodine-exposure history
- In subacute thyroiditis the gland is tender with very low RAIU despite thyrotoxicosis, distinguishing it from Graves
- Correlate every cold nodule with ultrasound and cytology, as scintigraphy alone cannot exclude malignancy
Structured Report
- State the tracer, administered activity, route and imaging/uptake timing
- Report the RAIU percentage with the reference range
- Describe gland size, position, symmetry and the pattern of uptake (diffuse versus nodular)
- Characterise nodules as hot or cold and recommend ultrasound/FNA for cold nodules
- Conclude on the likely cause of thyroid dysfunction and any therapy or follow-up implications
References
- SNMMI Procedure Standard / EANM Practice Guideline for Thyroid Scintigraphy and Uptake
- Giovanella L, et al. EANM practice guideline for thyroid scintigraphy and radioiodine uptake. Eur J Nucl Med Mol Imaging 2019
- Ross DS, et al. American Thyroid Association Guidelines for Hyperthyroidism. Thyroid 2016
- Meier DA, et al. SNM procedure guideline for therapy of thyroid disease with I-131
Educational clinical decision support only. Protocols vary by institution and equipment; always confirm with a qualified radiologist and local guidelines before clinical use.