# Parathyroid Scintigraphy > Parathyroid scintigraphy for NEET-PG Radiology: Tc-99m sestamibi dual-phase and subtraction protocols, adenoma localisation, pitfalls and the IOPTH Miami criterion. - Canonical URL: https://prepelephant.com/topics/neet-pg/radiology/parathyroid-scintigraphy-detail - Exam / course: NEET-PG · Subject: Radiology - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Parathyroid Scintigraphy", PrepElephant, https://prepelephant.com/topics/neet-pg/radiology/parathyroid-scintigraphy-detail ## Direct answer Parathyroid scintigraphy with Tc-99m sestamibi localises the hyperfunctioning gland behind primary hyperparathyroidism, almost always a solitary adenoma, before minimally invasive parathyroidectomy. In the dual-phase technique, early images at 10-15 minutes and delayed images at 2-3 hours exploit the fact that sestamibi washes out of normal thyroid tissue but is retained in mitochondria-rich adenoma cells, so a focus that persists on delayed imaging is the culprit; subtraction imaging combines sestamibi with Tc-99m pertechnetate thyroid images and subtracts the thyroid, leaving adenoma as residual activity. Sensitivity is high for solitary adenomas and falls in multiglandular hyperplasia, and in Indian practice — where the four-gland exploration remains common in many centres — localisation is what makes the minimally invasive, unilateral approach possible. ## What you must remember - **Mechanism:** sestamibi concentrates in mitochondria; oxyphil-rich parathyroid adenomas retain it while thyroid washes out — the entire test rests on differential washout, not on preferential uptake alone. - **Dual-phase protocol:** early phase 10-15 minutes, delayed phase 2-3 hours; a focus that persists or intensifies on delayed images marks the adenoma. - **Subtraction protocol:** sestamibi minus pertechnetate (or iodine-123) thyroid image; residual focus after subtraction is adenoma — superior in multinodular thyroids where dual phase struggles. - **Performance:** sensitivity around 90 per cent or better for solitary adenoma, but notably lower for multigland disease, small adenomas and secondary hyperparathyroidism (hyperplasia) — a negative scan does not exclude disease. - **Ectopic glands:** the exam reason for scintigraphy — adenomas hide in the thymus, mediastinum, retro-oesophageal space and carotid sheath; SPECT/CT gives 3D localisation that planar imaging cannot. - **Pitfalls:** thyroid adenoma or Hürthle-cell nodule retains sestamibi (false positive); cystic or small parathyroid adenomas and multigland disease miss (false negative); brown tumours and active thyroid malignancy can also take up tracer. - **Surgery pairing:** intraoperative PTH monitoring by the Miami criterion — a drop of over 50 per cent from baseline at 10 minutes after excision — predicts cure, enabling focused exploration. ## Localising one gland, stepwise A 48-year-old woman has renal stones, fatigue and serum calcium of 11.8 mg/dL with an intact PTH of 120 pg/mL, and ultrasound of the neck is negative. She is a candidate for minimally invasive parathyroidectomy if localisation succeeds. Sestamibi is injected, and the early image shows uniform thyroid uptake with a subtle inferior pole prominence; the 3-hour delayed image shows the thyroid faded and a discrete focus of retained activity below the right thyroid lobe, confirmed on SPECT/CT as lying posterior to the lobe — a right inferior adenoma. Surgery proceeds through a 2.5 cm incision, the adenoma is removed, and IOPTH falls from 120 to 40 pg/mL at 10 minutes — over 50 per cent, cure predicted, and no further exploration needed. The second scenario the exam loves: the same protocol in a patient whose focus sits in the superior mediastinum on SPECT/CT — an ectopic gland descended with the thymus — which converts the operation from a neck exploration to a coordinated thoracic approach, the exact situation scintigraphy exists to prevent discovering on the table. ## Where students slip Two mistakes dominate. The first is treating scintigraphy as a diagnostic test for hyperparathyroidism: the diagnosis is biochemical (raised calcium with inappropriately raised or non-suppressed PTH), and imaging only answers "which gland" in a patient already booked for surgery — scanning a patient not fit for surgery is pointless. The second is forgetting the thyroid confounder: in endemic goitre regions of India, multinodular thyroids retain sestamibi patchily, so the subtraction technique or additional ultrasound correlation is needed before calling a focus parathyroid. A viva favourite asks why sestamibi rather than the older thallium-pertechnetate subtraction — better retention and dosimetry — and another asks about secondary hyperparathyroidism of chronic kidney disease, where four-gland hyperplasia localises poorly and surgery is planned on bilateral neck exploration rather than a scintigraphic map. ## Frequently asked questions ### Why does a parathyroid adenoma retain sestamibi on delayed imaging? Adenomatous parathyroid cells are mitochondria-rich, and sestamibi is retained in mitochondria while it washes out of normal thyroid tissue over 2-3 hours. ### What are the two main parathyroid scintigraphy protocols? Dual-phase imaging with early 10-15 minute and delayed 2-3 hour acquisitions, and subtraction imaging in which a pertechnetate or iodine-123 thyroid image is subtracted from sestamibi to reveal a residual adenoma focus. ### In which situations is sestamibi scintigraphy less reliable? Multiglandular hyperplasia, small or cystic adenomas, secondary hyperparathyroidism of chronic kidney disease, and coexisting multinodular goitre, which both misses and mimics disease. ### Where are ectopic parathyroid adenomas commonly found? Intrathymic and anterior mediastinal, retro-oesophageal, within the thyroid parenchyma, and along the carotid sheath — SPECT/CT is used for precise 3D localisation. ### What is the Miami criterion in parathyroid surgery? A fall of intraoperative PTH by more than 50 per cent from the pre-excision baseline at 10 minutes after adenoma removal, predicting cure and allowing the operation to end.