Adrenal Cortical Scintigraphy (NP-59)¹³¹I-6β-iodomethyl-norcholesterol (NP-59)
Iodocholesterol imaging to lateralize functional adrenocortical disease
Quick Answer
Adrenal cortical scintigraphy uses NP-59 (I-131-6β-iodomethyl-norcholesterol), a cholesterol analog incorporated into steroid-synthesizing adrenocortical cells, to image the function of the adrenal cortex — complementing the anatomy from CT/MRI and the biochemistry that makes the diagnosis. Its classic role is lateralizing functional disease: distinguishing a unilateral aldosterone- or cortisol-producing adenoma (focal uptake) from bilateral hyperplasia (bilateral uptake). For cortisol/aldosterone work-ups, dexamethasone suppression is used to unmask autonomous cortical function. It is now largely supplanted by adrenal venous sampling (AVS) for aldosteronism and by CT/MRI, but remains a functional problem-solver.
Background & Mechanism
NP-59 follows the LDL/cholesterol pathway into adrenocortical cells and is esterified but not further metabolized, accumulating in proportion to cortical steroidogenic activity (which is ACTH- and, for aldosterone, angiotensin-driven). Because uptake reflects function, a hyperfunctioning adenoma concentrates tracer while the contralateral (suppressed) gland is faint — the basis of lateralization.
Dexamethasone Suppression (for cortisol/androgen work-ups)
For hypercortisolism/hyperandrogenism, dexamethasone is given to suppress ACTH-driven normal cortical uptake, so that only autonomous (ACTH-independent) tissue lights up:
- Early, unilateral uptake (before day ~5) during suppression → autonomous adenoma.
- Bilateral uptake → bilateral hyperplasia (e.g., ACTH-dependent or bilateral autonomous disease).
Thyroid blockade (stable iodine) is required (I-131 label), and bowel-activity mitigation aids interpretation.
Interpretation by Syndrome
| Setting | Focal (unilateral) uptake | Bilateral uptake |
|---|---|---|
| Primary aldosteronism (Conn) | Aldosterone-producing adenoma | Bilateral hyperplasia |
| Cushing (adrenal, ACTH-independent) | Cortisol-producing adenoma | Bilateral hyperplasia / macronodular disease |
| Incidentaloma | Functioning nodule | Non-lateralizing |
- Discordant uptake (a CT nodule that is non-functioning / photopenic) suggests a non-secreting mass or carcinoma (adrenocortical carcinomas often do not concentrate NP-59 well).
Contemporary Role
- Primary aldosteronism: adrenal venous sampling (AVS) is the reference standard for lateralization; NP-59 is a non-invasive alternative where AVS is unavailable/unsuccessful.
- Incidentaloma/Cushing: functional characterization when biochemistry and CT/MRI are discordant.
- Adrenocortical carcinoma: non-uptake of a large mass supports a non-functioning/malignant lesion (FDG-PET better characterizes malignancy).
Preparation & Protocol
- Thyroid blockade (stable iodine) before and after; bowel prep for abdominal clarity.
- Dexamethasone suppression for cortisol/androgen questions (per protocol).
- Delayed imaging over several days (I-131, cholesterol kinetics); SPECT/CT for localization.
Common Pitfalls
- Inadequate thyroid blockade (thyroid/gastric activity) or bowel activity confounding.
- Expecting uptake in adrenocortical carcinoma (often non-avid → "discordant" photopenic mass).
- Using it where AVS (aldosteronism) is the accepted standard.
- Misreading suppression timing (early bilateral vs unilateral) without protocol adherence.
Board Pearls
NP-59 (I-131-iodocholesterol) images adrenocortical function by incorporating into steroid-synthesizing cells — lateralizing a unilateral hyperfunctioning adenoma (focal uptake) from bilateral hyperplasia. For cortisol/androgen work-ups, dexamethasone suppression unmasks autonomous (ACTH-independent) uptake; thyroid blockade is mandatory (I-131 label).
A CT nodule that is NP-59 photopenic/non-functioning (discordant) suggests a non-secreting mass or adrenocortical carcinoma (which typically doesn't concentrate the tracer) — FDG-PET better assesses malignancy. NP-59 characterizes function, not malignancy.
For primary aldosteronism, adrenal venous sampling is the lateralization reference standard; NP-59 is a non-invasive alternative when AVS is unavailable. Imaging is delayed over several days (cholesterol kinetics, I-131), with SPECT/CT for localization and bowel prep to reduce confounding activity.
Related Pages
- Disease: Adrenal / pheochromocytoma (medullary counterpart: MIBG); tracer: MIBG (medulla vs cortex contrast).
Figure / Diagram Suggestions
- A cortex (NP-59) vs medulla (MIBG) adrenal-imaging contrast.
- A dexamethasone-suppression interpretation flow (unilateral vs bilateral uptake).
Self-Check
Q1. What does NP-59 image, and by what mechanism does it accumulate?
Answer: Adrenocortical function — NP-59 is a cholesterol analog incorporated into steroid-synthesizing cortical cells in proportion to steroidogenic activity.
Q2. Why is dexamethasone given for a cortisol/androgen NP-59 study?
Answer: To suppress ACTH-driven normal uptake so only autonomous (ACTH-independent) tissue concentrates tracer — early unilateral uptake = adenoma.
Q3. A CT adrenal mass is photopenic (non-avid) on NP-59. What does this suggest?
Answer: A non-functioning mass or adrenocortical carcinoma (often NP-59 non-avid) — FDG-PET better characterizes malignancy.
Q4. What is the reference standard for lateralizing primary aldosteronism, and where does NP-59 fit?
Answer: Adrenal venous sampling (AVS); NP-59 is a non-invasive alternative when AVS is unavailable or unsuccessful.
Evidence & sources
https://nuc.med/adrenal-cortical-scintigraphy/ Report an issue