**4.4 Secondary hyperparathyroidism**

Secondary hyperparathyroidism is characterized with hyperplasia of parathyroid glands, because it is caused by longstanding uncontrolled hypocalcemia, which leads to a profound overstimulation of a previously normal parathyroid glands. Over time this overstimulation causes hyperplasia and eventually adenomatous changes (tertiary hyperparathyroidism) of the parathyroid glands with PTH levels far more exceeding those observed in PHPT (**Figure 11)**. Nevertheless, hyperplastic parathyroid glands usually show faster wash out of the radionuclides in comparison to solitary adenomas, which makes them more difficult to be visualized with scintigraphy (**Figure 12)**. Negative scans, may be associated with the possible suppression of the accumulation of radiopharmaceuticals in the parathyroid cells as a result of the concomitant calcitriol intake. The use of calcium channel blockers may affect the uptake of 99mTc-sestamibi by parathyroid cells and reduce the sensitivity of the method. A study found that negative scans are twice as likely in patients taking calcium antagonists than those who do not take these medications (OR2, 88.95% CI, 1.03–8.10, p 0.045) [58]. So, adding the poor general condition of the patients, pathologically changed parathyroid glands are more difficult to be localized in SHPT than in PHPT.
