Parathyroid glands are overactive
(often due to a tumor) and secrete excess parathyroid hormone

Removal or destruction of tumor

Increased bone breakdown and resorption triggered by cancer cells causes higher levels of calcium in blood

Depends on specific type of cancer

Exact mechanism is unknown

Vitamin D supplements taken in excess
(e.g., to treat hypoparathyroidism); accidental ingestion of human psoriasis medicine or rodent poisons containing calcitriol

Adjustment of vitamin D dosage
(if from medication)

Inflammation activates type of white blood cells that can increase levels of active vitamin D

Treatment of underlying disease,
(e.g., antifungal drugs or surgery)