Why is it said that thiazide diuretics may unmask hypercalcemia?

Thiazide diuretics increase the risk of hypercalcaemia. There are several mechanisms by which they have been reported to contribute to increased serum calcium levels, but most important is that they increase renal tubular reabsorption of calcium resulting in reduced calcium excretion in urine. Normally, this is not a problem for otherwise healthy individuals. However, thiazide diuretics are contraindicated in patients at risk of hypercalcaemia, for example, because of primary hyperparathyroidism or sarcoidosis.

Primary hyperparathyroidism is the most common cause of hypercalcaemia. Excessive secretion of parathyroid hormone (PTH), a key factor in calcium metabolism, results in the leaching of calcium from bone and increased production of calcitriol, the active form of Vitamin D, which increases intestinal absorption of calcium and reabsorption in the kidneys. Sarcoidosis is clinically observed to be associated with increased risk of hypercalcaemia, which is most frequently explained by overproduction of calcitriol by activated macrophages.