Parathyroid Scintigraphy
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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.