12.2 Endocrine Imaging (Thyroid, Parathyroid, Adrenal)
Key Takeaways
- I-123 NaI is preferred for diagnostic thyroid morphology; Tc-99m pertechnetate shows trapping only (not organification); I-131 is used for uptake measurements and diagnostic/therapeutic whole-body pathways at very different activities
- Thyroid cancer whole-body surveys use diagnostic I-123 or low-activity I-131 with TSH stimulation (thyroid hormone withdrawal or rhTSH), low-iodine diet, and strict separation from therapy-level I-131 doses
- Parathyroid adenoma imaging classically uses Tc-99m sestamibi dual-phase washout and/or pertechnetate or I-123 subtraction; SPECT/SPECT-CT localizes ectopic or multigland disease
- Adrenal/sympathetic tumor imaging uses I-123 MIBG (norepinephrine analog) with mandatory thyroid blockade and interfering-medication holds; NP-59 is a historical cholesterol-analog adrenocortical agent
- High-yield traps: recent iodinated contrast or amiodarone suppressing uptake, skipped thyroid blockade for MIBG/free iodide, dual-phase sestamibi false positives from thyroid nodules, and confusing scan-dose vs therapy-dose radioiodine
12.2 Endocrine Imaging (Thyroid, Parathyroid, Adrenal)
Quick Answer: Thyroid: I-123 (preferred diagnostic imaging) vs Tc-99m pertechnetate (trapping only) vs I-131 (uptake / whole-body / therapy dose tiers). Parathyroid: sestamibi dual-phase ± subtraction, finish with SPECT/CT. Adrenal/sympathetic: I-123 MIBG + thyroid blockade; know NP-59 as legacy cortical imaging. Prep (iodine load, meds, TSH, blockade) makes or breaks the study.
Thyroid Imaging and Uptake
Physiology review
Follicular cells trap iodide (and pertechnetate) via the sodium-iodide symporter (NIS). Organification incorporates iodine into thyroglobulin/hormone pathways—I-123/I-131 participate; Tc-99m pertechnetate is trapped but not organified. Salivary glands and gastric mucosa also show NIS-related activity.
| Agent | What it shows | Typical diagnostic use |
|---|---|---|
| I-123 NaI | Trapping + organification; excellent 159 keV imaging | Morphology, nodules, uptake, preferred diagnostic scan |
| Tc-99m pertechnetate | Trapping map only | Quick thyroid morphology when iodine unavailable; discordant nodule workups |
| I-131 NaI (low activity) | Uptake quantification; delayed whole-body capability | Uptake %; diagnostic whole-body cancer survey |
Nodule language (exam favorites)
| Pattern | Teaching meaning |
|---|---|
| Hot (hyperfunctioning) nodule | Suppresses surrounding tissue; lower cancer probability but still manage clinically |
| Cold nodule | Decreased function; higher concern—correlate ultrasound/FNA |
| Discordant nodule | Hot on pertechnetate, cold on radioiodine → failed organification concern |
Uptake measurement
A neck probe or camera ROI measures percent uptake at protocol times (commonly 4–6 h and/or 24 h for iodine). Elevated uptake supports hyperthyroidism patterns (Graves, toxic nodule) when matched to labs; low uptake with thyrotoxicosis suggests thyroiditis or exogenous hormone. Recent iodinated IV contrast, amiodarone, iodine antiseptics, or high-iodine diet suppress measured uptake—document the exposure date.
Patient prep for diagnostic thyroid studies
| Prep item | Reason |
|---|---|
| Hold levothyroxine / liothyroxine or antithyroid drugs per order | Alters uptake and TSH drive |
| Avoid recent iodinated contrast (weeks–months effect) | Floods iodine pool |
| Pregnancy/breastfeeding policies | Absolute safety gates |
| NPO briefly for oral capsule only if protocol requires | Consistent absorption |
Whole-Body Survey for Thyroid Metastases
After near-total thyroidectomy for differentiated thyroid cancer, diagnostic whole-body imaging maps remnant and iodine-avid mets (neck nodes, lungs, bone).
| Element | Teaching standard |
|---|---|
| Tracers | I-123 whole-body or diagnostic I-131 (often ~1–5 mCi range—protocol/regulatory dependent) |
| TSH stimulation | Thyroid hormone withdrawal (hypothyroid TSH rise) or rhTSH injections |
| Low-iodine diet | Usually ~1–2 weeks before dosing for surveys/therapy planning |
| Imaging times | I-123 often ~24 h; I-131 diagnostic may include 48–72 h views |
| Therapy I-131 | Much higher activity under written directive—never call a therapy dose a “scan dose” |
Stunning (high-yield): larger diagnostic I-131 activities may reduce subsequent therapy uptake—many centers prefer I-123 surveys or minimize diagnostic I-131. Follow local protocol.
Traps: physiologic salivary/stomach/bowel/bladder activity; urine or saliva contamination; lactation breast uptake—survey gown/skin for unexpected foci.
Parathyroid Imaging
Primary hyperparathyroidism workups seek a hyperfunctioning adenoma (less often hyperplasia or carcinoma) before minimally invasive surgery.
Dual-phase sestamibi
Tc-99m sestamibi is taken up by both thyroid and abnormal parathyroid tissue (mitochondria-rich adenomas). Early images (~10–15 min) show thyroid + parathyroid; delayed images (~1.5–2.5 h) rely on faster thyroid washout so adenoma persists (“differential washout”).
| Method | Idea |
|---|---|
| Dual-phase sestamibi | Early vs delayed persistence at adenoma site |
| Subtraction | Sestamibi minus Tc-99m pertechnetate or I-123 thyroid map isolates parathyroid focus |
| SPECT / SPECT-CT | Depth, ectopic mediastinal glands, multigland disease, anatomic roadmap |
Typical adult activity is often ~20–25 mCi sestamibi IV (lab-specific). SPECT/CT is preferred when available for surgical localization.
Traps: thyroid nodules can retain sestamibi (false positive); small hyperplastic glands may be missed (false negative); recent IV contrast CT is complementary but not a substitute for functional localization. Always correlate calcium, PTH, and ultrasound.
Adrenal Imaging Concepts
I-123 MIBG (iobenguane)
MIBG is a norepinephrine analog taken up by the norepinephrine transporter into adrenergic tissue—pheochromocytoma, paraganglioma, neuroblastoma, and related tumors. Thyroid blockade (SSKI, Lugol’s, or perchlorate per protocol) is mandatory. Hold interfering drugs (selected antidepressants, labetalol, sympathomimetics, cocaine, others per insert) to avoid false-negative studies. Imaging typically includes 24-hour planar ± SPECT/CT; adult activity often ~10 mCi IV order of magnitude.
NP-59 (historical adrenocortical agent)
I-131 NP-59 (iodomethylnorcholesterol) is a cholesterol analog historically used to image the adrenal cortex (e.g., adenoma vs hyperplasia patterns in hyperaldosteronism/Cushing workups) with multi-day imaging and dexamethasone suppression variants. Modern practice often favors CT/MRI and venous sampling, but CNMT items may still expect: cortical cholesterol pathway, multi-day imaging, and dexamethasone suppression protocol concept. Do not confuse NP-59 (cortex) with MIBG (medulla/sympathetic).
| Pathway | Agent class | Target |
|---|---|---|
| Adrenal medulla / sympathetic tumors | MIBG | NET uptake storage granules |
| Adrenal cortex (legacy) | NP-59 | Cholesterol analog uptake |
Endocrine Prep and Trap Matrix
| Study | Must-do prep | Classic failure mode |
|---|---|---|
| Thyroid uptake/scan | No recent iodine load; correct med holds | Falsely low uptake after contrast/amiodarone |
| Cancer whole-body I-123/I-131 | TSH stimulation + low-iodine diet | Residual diet iodine → poor lesion visualization |
| Parathyroid sestamibi | Early + delayed ± SPECT/CT; know thyroid disease | Hot thyroid nodule called “adenoma” |
| MIBG | Thyroid blockade + med reconciliation | Free iodide in thyroid; drug-related false negative |
Selection cheat sheet: morphology with low dose → I-123; trapping-only quick look → pertechnetate; iodine-avid cancer map → stimulated whole-body iodine; PTH adenoma localization → sestamibi SPECT/CT; pheo/neuroblastoma → MIBG.
Why might a technologist prefer I-123 over Tc-99m pertechnetate when evaluating a thyroid nodule’s organification behavior?
Which preparation package is most appropriate before a diagnostic radioiodine whole-body survey for differentiated thyroid cancer?
In dual-phase Tc-99m sestamibi parathyroid imaging, what is the expected behavior of a typical parathyroid adenoma relative to normal thyroid tissue?