Factors leading to decreased 1,25(OH)₂D levels

Decreased renal 1α-hydroxylase activity, leading to decreased renal parathyroid hormone responsiveness

Decreased vitamin-D synthesis by the skin

Decreased sunlight exposure (housebound and institutionalized elderly persons)

Factors leading to decreased intestinal absorption of dietary calcium

Inadequate dietary calcium and vitamin-D intake

Decreased intestinal responsiveness to 1,25(OH)₂D

Decreased gastric acid secretion

Lactase deficiency (avoidance of dairy products)

Factors leading to age-related increase in serum parathyroid hormone levels

Slight decrease in serum calcium levels

Decreased renal clearance of parathyroid hormone

Decreased 1,25(OH)₂D levels